Lessons learned from the DIASTAT vascular access graft

In male Syrian hamsters, short days induce regression of the reproductive system, but eventually spontaneous recrudescence occurs ensuing from refractoriness to the inhibitory photoperiod. Although the photoperiod of 12 L:12 D (12 h light:12 h dark) may act like a short day by inducing the testicular cycle outlined above, it may fail to evoke the increase of circulating concentrations of prolactin that accompanies testicular recrudescence. This photoperiod may fail to induce photorefractoriness, as indicated by the prolonged low concentrations of prolactin in the blood. Herein, hamsters were exposed to either 8 L:16 D or 12 L:12 D from weaning and by 28 weeks exposure to either photoperiod, the hamsters had large testes following recrudescence from a photoperiod induced-regression). Transfer to 8 L:16 D from 12 L:12 D at 28 weeks resulted in a second testicular regression and recrudescence. In a second experiment, the testes of hamsters moved to 8 L:16 D from 12 L:12 D after 29 weeks exposure to the latter photoperiod similarly regressed and then regrew. Serum concentrations of prolactin decreased in these males after transfer to the shorter daylength but also decreased in hamsters kept under 12 L:12 D both groups were usually below those of hamsters moved to 14 L:10 D. These results show that exposure for 28 or 29 weeks to 12 L:12 D was insufficient to induce photorefractoriness, despite the recrudescence of the testes following involution.     

Flow-injection spectrophotometric determination of diclofenac sodium in pharmaceuticals and urine samples

The localization of two salmon-type gonadotropin-releasing hormone (sGnRH) precursors, pro-sGnRH-I (short type) and pro-sGnRH-II (long type), was investigated by using in situ hybridization techniques in the brain of the landlocked sockeye salmon, Oncorhynchus nerka. We used 30-mer oligonucleotide probes complementary to pro-sGnRH-I and pro-sGnRH-II cDNA. No significant differences were observed in the localization of sGnRH neurons expressing pro-sGnRH-I and pro-sGnRH-II mRNAs; both were expressed in the olfactory nerve, the olfactory bulbs, the regions between the olfactory bulb and telencephalon, the ventral telencephalon, the preoptic area, and the hypothalamus. Almost all sGnRH neurons examined co-expressed both precursors. The expression of two sGnRH precursors in the same neuron and the wide distribution of such neurons in the brain suggest that there are no functional differences between the two precursors.     

Influence of photoperiod and social environment on sexual maturation in female deer mice (Peromyscus maniculatus bairdii).

Tested the reproductive response of young female deer mice to either a long or short photoperiod combined with the presence of adult males, adult females, or social isolation. 266 Ss were reared on either a long photoperiod (15 hrs light, 9 hrs dark) or on short days (8 hrs light, 16 hrs dark) from birth. Beginning at weaning, females were housed with an adult male, with an adult female, or in social isolation. In Exp I, vaginas opened more slowly in Ss on short days than in those on long days. Vaginal introitus was also retarded in Ss reared with an adult female in comparison with Ss reared in isolation. When examined at 37 days of age, Ss reared with an adult male had larger uteri than those reared alone; uteri were also larger in long-day than in short-day Ss. In Exp II, Ss were killed at 30 days of age; again, uterine growth was stimulated by exposure to either long days or an adult male. As was previously demonstrated for male deer mice, sexual maturation in females is regulated by photoperiod and social cues. Heterospecific social stimuli accelerate maturation in individuals that otherwise would be inhibited by having been reared on a short photoperiod. (27 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dual origins of Finns revealed by Y chromosome haplotype variation

The objective of this study was to determine effects of photoperiod and age on the circulating concentrations of LH, testosterone (T), thyroxine (T4), and semen production in male turkeys. Male turkeys from 10 or 12 wk of age were maintained under either a long-day (LD) photoperiod of 16L:8D up to 35 wk (LL), or a short-day (SD) photoperiod of 6L:18D to 29 wk and then an LD photoperiod of 16L:8D up to 35 wk of age (SL). Plasma concentrations of both LH and T increased by 17 or 18 wk of age under both photoperiods, but higher levels were attained in the LL group prior to 29 wk of age. Both LH and T levels increased significantly within days in the SL group after the group was switched to LD at 29 wk of age. Higher levels of T4 were present in the LL group prior to sexual maturation. No differences were observed in T4 concentration between lighting treatments after sexual maturation. The LL group first produced semen at 20-22 wk of age, which was about 1 wk earlier than first semen production in the SL group. A significantly larger volume of semen was produced in the LL group at most ages. No further increase in semen production was observed in the first 6 wk after the SL group was switched to the LD photoperiod at 29 wk of age. Pulsatile patterns of LH and T were characterized by serial blood sampling at 13, 23, and 35 wk of age under both the LL and SL photoperiods. The baseline levels of both LH and T in male turkeys were influenced by age and photoperiod. However, pulse characteristics (numbers, duration, and amplitude) of LH did not change with age or lighting treatment, while pulse characteristics of T did change with age. We conclude that exposing male turkeys to an LD photoperiod from 10 or 12 wk of age advanced the age of sexual maturation and induced earlier increased concentrations of LH and T.     

Kinetics of humoral response in children with acute lymphoblastic leukemia immunized with influenza vaccine in 1993 in Poland

Songbirds exhibit seasonal changes in the volumes of song control nuclei. Birds on long, spring-like days have larger nuclei than do birds on short, winter-like days. The mechanisms mediating volumetric changes have not been determined unequivocally, but testosterone (T) is probably involved. This study examined whether testicular factors are uniquely responsible for seasonal changes in the song system, or whether photoperiod has testis-independent effects. Male American tree sparrows were exposed to one of three photoperiodic conditions: (1) Photosensitive birds were retained on short days (8L:16D). Plasma T is rarely detected in such birds. (2) Photosensitive birds were moved from short days to long days (20L:4D) and photostimulated for three weeks. Photostimulation elevates circulating T in photosensitive birds. (3) Photorefractory birds were held at least four months on 20L:4D. Such birds seldom have detectable levels of T, even though they are on long days. In each condition, there were both intact and castrated birds. Castration typically removes circulating T in tree sparrows. The volumes of the high vocal center (HVC), nucleus robustus archistriatalis (RA), and area X were measured. Song nuclei were largest in intact photostimulated birds. Other long-day birds (i.e. castrated photostimulated, and intact and castrated photorefractory groups) had larger song nuclei than did short-day intact or castrated photosensitive birds and did not differ from each other. These data indicate that photoperiod has both testis-dependent and -independent effects on the volumes of song control nuclei.     

Effects of photoperiod on reproduction and the gonadotropin-releasing hormone-immunoreactive neuron system in the postpubertal male Djungarian hamster

The present study tested the hypothesis that photoperiodic control of reproductive function in the postpubertal Djungarian hamster is associated with changes in the number, morphology, or distribution of GnRH-immunoreactive cell bodies in the brain. To initiate or arrest sexual maturation, males were reared in long (LD, 16L:8D) or short (SD, 10L:14D) days from birth. In two other groups that were chronologically past the normal onset of puberty, males were transferred at 30 days of age from LD to SD or from SD to LD to arrest or initiate reproductive function, respectively. At 40, 60, or 90 days of age, 4-6 hamsters in each of the four photoperiod treatment groups were killed by intracardiac perfusion. Testes weights were significantly increased in males exposed to long days (LD and SD-to-LD groups) compared to those treated with short days (SD and LD-to-SD groups). Serum FSH concentrations at 40 days of age were also increased in the two groups of males in long days compared to those in both groups in short days (p < 0.05, ANOVA); LH concentrations were unaffected by photoperiod treatments. Brain sections (60 microns) from the corpus callosum decussation to the suprachiasmatic nucleus in the anterior hypothalamus were processed for GnRH immunocytochemistry. In brain regions that contained the majority of GnRH neurons, i.e., the medial preoptic area and diagonal band of Broca, the numbers of GnRH-immunoreactive cell bodies were the same among the four treatment groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fosinoprilate prolongs the action potential: reduction of iK and enhancement of the L-type calcium current in guinea pig ventricular myocytes

Changes in the gross and cellular morphology of the nucleus preopticus medianus (POMn) were measured in response to changes in photoperiod in adult male Japanese quail (Coturnix japonica). POMn volume and the soma size of a dorsolateral population of neurons within POMn decreased when birds were moved from long day housing conditions (16L,8D) to short day housing conditions (8L,16D), and then increased again when birds were moved back to long day conditions, presumably as a function of the changes in circulating testosterone that accompanied changing daylengths. Male Japanese quail exhibit sexual behavior only when housed under long day housing conditions that approximate the photoperiod of the spring/summer breeding season, and do not exhibit sexual behavior when housed under short day conditions characteristic of fall/winter. Because POMn is known to be critically involved in the expression of male copulatory behavior, these morphological changes in the adult brain likely represent key functional events associated with the seasonal regulation of sexual behavior in male Japanese quail.     

Olfactory bulbectomy induces reproductive recrudescence in golden hamsters on short photoperiod

Olfactory bulbectomy results in a marked increase in gonadotropin secretion and prevents the reproductive regression associated with short photoperiod when the olfactory bulbectomy is done before exposure to the inhibitory photoperiod. The present study tested whether olfactory bulbectomy would offset the influence of short photoperiod if done after the reproductive system had regressed. Adult golden hamsters Mesocricetus auratus were divided into four groups: early sham (surgery at week-4); early olfactory bulbectomy (surgery at week-4); late sham (surgery at week 14) and late olfactory bulbectomy (surgery at week 14). At t = 0, all golden hamsters were placed in a short photoperiod (L:D 10:14). Early olfactory bulbectomy prevented testicular regression; the late olfactory bulbectomy group recrudesced much earlier than the sham groups. These results indicate that the tonic inhibitory influence of the olfactory bulbs is required for initiation of short photoperiod induced testicular regression and is also essential for the maintenance of the regressed state.     

Exposure to constant photoperiod alters serum prolactin concentrations and behavioral response to estradiol in the ovariectomized goat

The effects of constant photoperiod on serum prolactin concentrations and sexual behavior of ovariectomized goats was investigated. Fourteen ovariectomized goats were divided into groups of seven goats each and placed in photoperiod-controlled rooms with either 8L:16D or 16L:8D. All goats received six estradiol (E2) treatments in a Latin square design. Following each treatment six 1-h observation periods were conducted. Blood was collected before E2 treatment and at each observation period for prolactin quantification. During the observation periods measures of attractivity, proceptivity, and receptivity were recorded. Goats exposed to 8L:16D displayed sexual behavior in response to doses of E2 lower than those required by goats exposed to 16L:8D. Prolactin concentrations were higher in goats exposed to 16L:8D than in those exposed to 8L:16D. It was concluded that photoperiod affects prolactin concentrations and behavioral sensitivity to E2 in goats. The effect of continued exposure to constant photoperiod on these variables was then investigated. The goats were held in constant photoperiod for 211 d and behavior tests as described for Exp. 1 were repeated. Scores for attractivity, proceptivity, and receptivity did not differ between the two groups after 211 d of exposure to constant photoperiod. Prolactin concentrations did not differ in this study between the two groups. It was concluded that after chronic exposure to 8L:16D ovariectomized goats become refractory to the stimulatory effects of short photoperiod.     

Photoperiodic control of gonadotrophin secretion in the ram: a detailed study of the temporal changes in plasma levels of follicle-stimulating hormone, luteinizing hormone and testosterone following an abrupt switch from long to short days

Six adult Soay rams were housed under artificial lighting conditions of long days (16 h light:8 h darkness) for 4 months and this caused the animals to lapse into a state of reproductive quiescence with low levels of gonadotrophins in the circulation and regressed testes secreting very low amounts of testosterone. The photoperiod was changed abruptly to short days (8 h light:16 h darkness) to induce a resurgence of sexual activity, and a detailed study was made of the pituitary and testicular responses over the first 100 days. Plasma levels of LH and FSH first began to increase between days 6 and 12 of short days, and rose progressively until days 33-54 before declining again. Testicular growth of the rams began on days 19-26 and continued for most of the remaining period of study. Plasma testosterone levels rose in parallel with the growth of the testes, and were greatly increased by day 100 when gonadotrophin levels were reduced. At most stages there were short-term fluctuations in the plasma levels of FSH, LH and testosterone indicative of episodic secretion. Peaks in plasma levels of LH were especially conspicuous and from the changes in frequency and amplitude of these peaks it was possible to predict the way in which photoperiod influenced gonadotrophin secretion by its effect on hypothalamic LH-RH secretion. A slight 24 h rhythm in the plasma levels of all three hormones was observed, and the significance of this in relation to the photoperiodic response is discussed.     

Effect of short photoperiodic cycles on male genital tract and testicular parameters in male goats (Capra hircus)

This study was performed in adult male goats in which seasonal variations were abolished by rapid alternations of long days and short days. These treatments have been shown previously to prevent seasonal changes in the hypothalamo-pituitary axis and to maintain testis weight and sperm production at a high level. The experimental groups were exposed for 3 years to an alternation of either a 1 month short (16 h dark; 8 h light) and 1 month long (16 L; 8 D) photoperiod (2 month cycle; n = 5) or of a 2 month short and 2 month long photoperiod (4 month cycle; n = 4). The control groups were maintained in natural photoperiodic conditions (45 degrees N) and goats were slaughtered in the non-breeding season (end of April RS; n = 5) at the same period as light-treated bucks, or in the breeding season (end of September BS; n = 6). The total weight of the testes, the length and mean diameter of the seminiferous tubules of light-treated goats were similar to those in the breeding season, and higher than those in the non-breeding season. The total number of A0 spermatogonia was increased by light treatments as compared to control goats in the breeding and non-breeding season. The daily production of A1 spermatogonia, leptonene primary spermatocytes and round spermatids in light-treated goats was maintained at the peak breeding season level. The intra-testicular concentration of testosterone, total volumes of intertubular tissue and of Leydig cells, and the number of Leydig cells per testis did not differ between groups. Although the mean cross-sectional area of Leydig cells in light-treated goats was similar to this area in non-breeding season goats, it was significantly lower than that of breeding season goats. In conclusion, the rapid alternation of short and long days allowed an increase in all the germ cells from the A0 spermatogonia onwards, which was responsible for the maintenance of high spermatogenetic activity of light-treated goats.     

Differential effects of short photoperiod on 2-[125I]iodomelatonin binding in the testis and brain of quail

The affinities and densities of 2-[125I]iodomelatonin binding sites in the brains and gonads of male Japanese quail following short photoperiod treatment were studied. At 6 weeks old, control quail were placed under a 14 hour light/10 hour dark photo-stimulatory cycle and experimental quail were housed under a 7 hour light/17 hour dark photo-inhibitory lighting regime. Eighteen weeks after photic manipulation, the birds were killed at mid-light. The photo-inhibited quail had very small testes. Brains and testes of control and experimental quail were collected for receptor binding studies. The maximum number of binding sites (Bmax) of 2-[125I]iodomelatonin determined by saturation studies and the number of 2-[125I]iodomelatonin binding sites determined by a one-point binding assay in the testes of short-day quail were significantly lower (p < 0.05) than those of the testes in reproductively active birds kept under long photoperiod. There was no significant difference (p > 0.05) between the testicular Kd (equilibrium dissociation constant) values of these two groups. As for the Kd and Bmax of 2-[125I]iodomelatonin binding sites in the whole brain, there were no significant differences (p > 0.05) between the two groups. The higher level of testicular 2-[125I]iodomelatonin binding sites in photostimulated birds may be related to an up-regulation of melatonin receptors by the suppressed pineal melatonin secretion under long photoperiod. The lower testicular 2-[125I]iodomelatonin binding sites under short photoperiod may be the result of down-regulation of melatonin receptors by the stimulated melatonin pattern in the photo-inhibited birds.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of estrogen and progesterone in the regulation of reproductive behaviour in female ring doves(Streptopelia risoria) under long vs. short photoperiods

The role of estrogen and progesterone in the regulation of reproductive behavior was studied in female ring doves (Streptopelia risoria) under long vs. short photoperiods. The doves held under long (16 hours/day) or short (8 hours/day) photoperiods were treated for 15 days with saline, estrogen, or progesterone, alone or in combination. 7 days after the start of hormone therapy, the doves were placed with reproductively active males for 4 hours/day for 9 days. No difference was seen in egg-laying, courtship or nest-building by control birds under the 2 photo regimes. Ovarian follicles remained small in all doves receiving hormone treatment. Oviducts of doves receiving progesterone alone remained small in the short-photoperiod group but at long photoperiods the oviducts became enlarged. Estrogen stimulated oviduct growth at both photoperiods wheras the combined hormone treatment resulted in greater oviduct development than estrogen alone (p less than .02). Female soliciting and nest-building activity remained low in progesterone-treated doves under short photoperiod but increased rapidly under long photoperiod. Doves treated with estrogen and estrogen plus progesterone performed considerable soliciting and nest-building. No marked tendency was seen for doves under long-photoperiod conditions to be more active in nest-oriented behavior. Copulatory behavior by the female occurred infrequently in all hormone-treated doves.     

Effect of photoperiod upon age and maintenance of sexual development in female Coturnix coturnix japonica

Coturnix kept in a 14L:10D photoperiod from hatch began to lay their first eggs at a mean age of 42.8 days (range 38-55). Approximately 2/3 of Coturnix held from hatch in photoperiods of 6L:16D light were laying at 165 days of age. Mean age at first egg was 112.7 days (range 68-162 days) in 8L:16D and 130.8 days (range 117-158 days) in 6L:18D photoperiod. Coturnix transferred from a non-stimulatory (8L:16D) photoperiod to a stimulatory one (14L:10D or 24L) begun laying in 15-20 days if less than 140 days old, and in about 5 days if greater than 140 days old, when trasferred. Birds which has spontaneously begun to lay in an 8L:16D photoperiod did not stop laying when the photoperiod was reduced to 6L:18D. Those which began laying under 14L:10D photoperiod ceased laying in about 15 days if 89 or fewer days old when switched to 8L:16D, or in about 6 days if 140 or more days old. Those switched from 14L:10D to 6L:18D ceased laying in about 13 days when 76 days old, and 7 days when 89 days old.     

Exon scrambling of MLL transcripts occur commonly and mimic partial genomic duplication of the gene

The aim of this study was to test the hypothesis that prolactin may up- and down-regulate prolactin receptor gene expression in the anterior pituitary gland and hypothalamus respectively. Experiments were carried out in bantams (Gallus domesticus). Comparisons were made of concentrations of PRLR mRNA in the anterior pituitary gland and basal and preoptic hypothalamus in adult males and females held on long days (low vs high plasma prolactin); in 3-week-old juvenile male and females on short days (high vs low plasma prolactin); in 8-week-old juvenile male and females on short days (both low plasma prolactin); in adult laying, incubating, and out-of-lay (high, very high, and low plasma prolactin, respectively); in adult cockerels exposed to long or short days (high vs low prolactin); and in adult hens exposed to long or short days (high vs low prolactin). There was a sex difference in anterior pituitary and basal hypothalamic PRLR mRNA, with lower values in both tissues in females than in males. Compared with laying and out-of-lay hens, anterior pituitary and basal hypothalamic PRLR mRNA concentrations in incubating hens were increased and decreased, respectively. In adult birds of either sex held on long or short days, there was no difference in pituitary PRLR mRNA, while basal hypothalamic PRLR mRNA was lower on short days. PRLR mRNA in the preoptic hypothalamus was not affected by sex, reproductive state, or photoperiod. It is concluded that there is no consistent relationship between plasma prolactin, in the physiological range, and the concentration of PRLR mRNA in the anterior pituitary gland, basal hypothalamus, and preoptic hypothalamus.     

The AEP T-complex to synthesised musical tones: left-right asymmetry in relation to handedness and hemisphere dominance

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, the seasonal breeding can be controlled by photoperiod, which affects the durations of nightly melatonin secretions. Winterlike short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo gonadal development much later than animals born into long days. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), comprise a sexually dimorphic, androgensensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8:16 h light/dark cycle) versus long-day (16:8 h light/dark cycle) conditions. At 40-47 days of age, development of three components of the SNB neuromuscular system were all significantly delayed in hamsters raised in the short photoperiod: BC/LA muscle weight, the size of SNB motoneuronal somata, and the area of the neuromuscular junctions at the BC/LA muscles of short-day hamsters were each significantly reduced relative to those of longday counterparts. Thus, development of the SNB reproductive system is delayed under short day lengths in this species.     

The influence of photoperiod and sex on lipopolysaccharide-induced hypoactivity and behavioral tolerance development in meadow voles (Microtus pennsylvanicus)

Lipopolysaccharide (LPS), the minimal immunogenic component of Gram-negative bacteria, is released during infection and causes a variety of sickness behaviors including decreased locomotor activity. This study considered how photoperiod and sex influence the effects of LPS in the meadow vole, Microtus pennsylvanicus. Male and female voles were housed under either reproductively stimulatory (long day: 16 h) or inhibitory (short day: 8 h) photoperiods. On Days 1 and 8, voles were injected with LPS (200 microg/kg, i.p.) or saline vehicle and locomotor activity was assessed 2 h later in an automated open field for 1 h. The first exposure to LPS caused significant decrements in locomotor activity in all LPS-treated groups, regardless of photoperiod or sex. On Day 8, both short day males and females exhibited behavioral tolerance to LPS, no longer displaying significant activity decrements. In contrast, long day females reinjected with LPS on Day 8 still exhibited significant hypoactivity on all locomotor measures. Similarly, long day males also appeared to exhibit a sustained expression of sickness behaviors on Day 8. In long day females, higher circulating progesterone levels were associated with an attenuated rate of tolerance formation to LPS. The present findings support the winter immunoenhancement hypothesis, which states that small mammals which undergo severe seasonal fluctuations undergo compromised immune functioning during the breeding season, and further indicate a potential role for progesterone in modulating these seasonal immune fluctuations in females.     

Secondarily infected wounds and dermatoses: a diagnosis and treatment guide

In golden hamsters, seasonal changes in day length act via a pineal-dependent mechanism to regulate feedback and behavioral effects of androgen. Endogenous opiates participate in photoperiodically regulated neuroendocrine functions, but the effects of androgen on expression of the gene encoding POMC, the precursor of beta-endorphin, have been controversial. We used quantitative in situ hybridization to examine regulation of POMC messenger RNA (mRNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) days for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and surgical treatment (castration and testosterone administration). Testosterone increased the number of silver grains in labeled cells throughout the arcuate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testosterone exerted an inhibitory effect, however, on the number of POMC mRNA-positive cells, and more POMC mRNA-labeled cells were found in the arcuate nucleus of long than short day castrates treated with testosterone. Photoperiod had no significant influence in castrates not receiving androgen. Testosterone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and the number of labeled cells. The results indicate that day length regulates POMC gene expression when androgen levels are held constant, but that androgen is necessary for photoperiod effects to be expressed.     

Seasonal and circadian changes in the episodic release of follicle-stimulating hormone, luteinizing hormone and testosterone in rams exposed to artificial photoperiods

Six rams of an ancient breed of domesticated sheep (SOAY) were subjected to an artificial light régime of alternating periods of long days (16 h light: 8 h darkness) and short days (8 h light: 16 h darkness) which induced seasonal development and regression of the testes during a period of 36 weeks. Over 2000 blood samples were taken, and the changes in plasma levels of FSH, LH and testosterone were related to the cycle of testicular activity. During long days plasma levels of gonadotrophins became very low and the testes regressed to about 20% of their maximum size; there was a corresponding reduction in plasma testosterone levels. When the rams were returned to short days reproductive development was again stimulated after 2-3 weeks with a progressive increase in plasma FSH and LH levels and consequent hypertrophy of the testes. It took about 16 weeks of short days for testicular activity to become maximal. Blood samples collected at hourly intervals for 24 h on ten occasions during the study revealed transitory peaks in plasma FSH and LH levels indicative of episodic release. Changes in gonadotrophin secretion were modulated primarily by alterations in the frequency of episodic releas; less than 1 spike per 24 h during long days increased to a maximum of 10 spikes/24 h under short daylengths. The peaks of FSH release were of smaller amplitude than those of LH, although during periods of frequent episodic release basal levels of fsh were increased to a greater extent than those of LH. A circadian rhythm was observed in the plasma levels of FSH, LH and testosterone, which was related to increased gonadotrophin release during the dark phase of the 24 h cycle; changes in blood haematocrit were also observed. The circadian changes appeared to be correlated with the activity cycle of the animals which in turn was dictated by daylight. A possible interrelationship between the circadian cycle and the seasonal cycle is discussed.