Plasma Sex Steroid Concentrations Research Articles

Serum concentrations of various environmental contaminants and their relationship to sex steroid concentrations and phallus size in juvenile American alligators.

Recent studies have reported a number of abnormalities in the hatchling and juvenile alligators of Lake Apopka, FL (USA). These abnormalities include modifications of plasma concentrations of sex steroids in males and females as well as abnormalities in gonadal morphology, gonadal enzyme activity, and steroidogenesis. Embryonic exposure to environmental contaminants in the eggs has been hypothesized to be the causal agent for these changes. However, posthatchling exposure can also contribute to changes in reproductive and endocrine functioning. We have detected serum concentrations of 16 of 18 organochlorine pesticides or metabolites (OCs) and 23 of 28 congener-specific polychlorinated biphenyls (PCBs) examined in juvenile alligators from Lake Apopka, Orange Lake, and Lake Woodruff National Wildlife Refuge. Lake Apopka juveniles had significantly elevated serum concentrations of p,p'-DDE, dieldrin, endrin, mirex, oxychlordane, SigmaDDTs, and SigmaPCBs compared to juveniles from the other lakes. Further, we observed no correlations between serum contaminant concentrations and sex steroid concentrations (estradiol-17beta and testosterone). However, serum testosterone was significantly lower in males from Lake Apopka and Orange Lake compared to Lake Woodruff NWR. We did not observe relationships between phallus size or other body parameters and serum contaminant levels. Phallus size was smaller in males from Lake Apopka even after adjustment for body size. We suggest that the observations previously reported for juvenile alligators-and observed again in this study-are apparently not associated with the current serum levels of the environmental contaminants we measured, but could be due to exposures during embryonic development to these or other pollutants. Future studies must determine if a causal relationship exists between the contaminants found in alligator eggs and abnormalities observed in the hatchlings and persisting in juveniles.

Plasma concentrations of sex steroids and fighting in male Tilapia zillii

In laboratory‐held Tilapia zillii the gonadosomatic index (IG) is an accurate predictor of dominance in pairwise fights between males. Immediately after fighting no differences were detected between winners, losers or controls, in plasma concentrations of testosterone (T) or 11‐ketotestosterone (11KT). 11KT was correlated negatively with IG and positively with T. 11KT appeared to be the major androgen in this species, but there was no evidence that simple differences in levels of these sex steroids mediated aggressive behaviour.

Plasma concentrations of sex steroids and fighting in maleTilapia zillii

Plasma concentrations of sex steroids and fighting in maleTilapia zillii

Sex steroids, growth and condition of Arctic charr broodstock during an annual cycle

Changes in plasma concentrations of sex steroids, growth rate and condition of repeat spawning (3+) male and female Arctic charr were studied throughout an annual reproductive cycle. Individually marked fish (mean weight approx. 500 g) were held under conditions of liberal food supply, constant temperature (4° C) and simulated natural photoperiod (Tromsø, 70° N). Once each month fish were weighed, measured and blood samples taken for steroid analysis. Plasma concentrations of testosterone (T), 11‐ketotestosterone (11‐KT) and oestradiol‐17β (E2) were determined using radioimmunoassay (RIA). Both male and female fish displayed distinct seasonal changes in plasma concentrations of sex steroids, growth rate and condition. From February (minimal concentrations) to March all sex steroids increased slightly and these elevated concentrations were maintained until May. Thereafter, there was a second, and far more pronounced, increase in plasma steroid concentrations which culminated in peak steroid concentrations in September–October. There was then a rapid decline during the spawning period. In winter, growth rate and condition were generally low, then increased during the spring, reached a peak during the summer, and then declined with the onset of autumn. During spring (March–May), the frequency distributions of plasma testosterone concentrations in both male and female fish were bimodal. The fish of the upper modal group of the distribution had significantly higher growth rates and condition than those in the lower modal group. In summer and early autumn (June–September) the association between T and growth rate changed. Significant negative correlations between T and growth rates were observed in females. There was an increase in endocrine activity, indicated by elevated plasma sex steroid concentrations in March, 7–8 months prior to maturation. It is suggested that this may be one factor influencing the onset of spring growth and energy deposition among maturing charr.

Sex steroids, growth and condition of Arctic charr broodstock during an annual cycle

Changes in plasma concentrations of sex steroids, growth rate and condition of repeat spawning (3+) male and female Arctic charr were studied throughout an annual reproductive cycle. Individually marked fish (mean weight approx. 500 g) were held under conditions of liberal food supply, constant temperature (4° C) and simulated natural photoperiod (Tromsø, 70° N). Once each month fish were weighed, measured and blood samples taken for steroid analysis. Plasma concentrations of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17β (E2) were determined using radioimmunoassay (RIA). Both male and female fish displayed distinct seasonal changes in plasma concentrations of sex steroids, growth rate and condition. From February (minimal concentrations) to March all sex steroids increased slightly and these elevated concentrations were maintained until May. Thereafter, there was a second, and far more pronounced, increase in plasma steroid concentrations which culminated in peak steroid concentrations in September–October. There was then a rapid decline during the spawning period. In winter, growth rate and condition were generally low, then increased during the spring, reached a peak during the summer, and then declined with the onset of autumn. During spring (March–May), the frequency distributions of plasma testosterone concentrations in both male and female fish were bimodal. The fish of the upper modal group of the distribution had significantly higher growth rates and condition than those in the lower modal group. In summer and early autumn (June–September) the association between T and growth rate changed. Significant negative correlations between T and growth rates were observed in females. There was an increase in endocrine activity, indicated by elevated plasma sex steroid concentrations in March, 7–8 months prior to maturation. It is suggested that this may be one factor influencing the onset of spring growth and energy deposition among maturing charr.

Investigation of the relationship between farrowing environment, sex steroid concentrations and maternal aggression in gilts.

Maternal oestrogen and progesterone have been shown to be important in the initiation of maternal behaviour. Thirty-three Large White x Landrace gilts, housed in groups during pregnancy, were observed and aggressive interactions recorded. Individuals had jugular catheters implanted 14.5 (s.e. 0.34) days before their expected parturition date (EPD). Five days before EPD gilts were randomly allocated and moved to either a conventional farrowing crate (C; without straw, 16 gilts) or a pen (P; 2.1 x 3.1 m2; with straw bedding, 17 gilts). Blood samples were taken at frequencies determined by the proximity to farrowing onset. Piglets were removed at birth and returned 2 h after placental expulsion. The reaction of each gilt to her piglets was monitored. Gilts savaging piglets were sedated with azaperone (n = 8). There was no overall effect of farrowing environment on oestradiol and progesterone concentrations. The pre-farrowing ratio of progesterone to oestradiol was higher for (P) gilts (0.45 vs. 0.25, (P) vs. (C); S.E.D. 0.085, P < 0.05) as was their overall maximum oestradiol level (3.39 vs. 2.29 ng/ml, (P) vs. (C); S.E.D. 0.39, P < 0.01). In contrast to progesterone, oestradiol patterns varied considerably between individuals. Dominance rank value during pregnancy, but not levels of aggression, correlated positively to pre-farrowing oestradiol concentrations. Treatment with azaperone was not related to farrowing environment, piglet weight or litter size. Azaperone treated gilts showed a higher pre-farrowing oestradiol to progesterone ratio (0.55 vs. 0.29, +/- azaperone; S.E.D. 0.10, P < 0.05), significantly higher levels of oestradiol post-partum (0.7 vs. 0.19 ng/ml, +/- azaperone; S.E.D. 0.20, P < 0.001) and significantly lower levels of aggression during pregnancy (1.68 vs. 2.23 aggressive interactions/h, +/- azaperone; S.E.D. 0.15, P < 0.001). The results indicate that there are no major effects of farrowing environment on sex steroid concentrations. Maternal aggression under these conditions appears to be negatively related to aggression during pregnancy, but this is not reflected in plasma concentrations of sex steroids around parturition.

Annual change in reproductive condition and plasma concentrations of sex steroids in black bream, Acanthopagrus butcheri (Munro) (Sparidae)

Changes in gonadosomatic index (GSI), hepatosomatic index (HSI), gonad stage and plasma concentrations of sex steroids were studied over one year in black bream (Acanthopagrus butcheri). Black bream have an annual reproductive cycle with a 3-month spawning season in spring–early summer. GSI and HSI values were highest in October and May respectively. Plasma concentrations of oestradiol-17β (E2), testosterone (T) and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were highest in females in October. Plasma concentrations of E2 and T were highest in ovulated fish. Concentrations of 17,20βP were higher in fish undergoing final oocyte maturation (FOM) than in fish with regressed gonads. In males, plasma concentrations of T and 11-ketotestosterone (11KT) increased in September and remained elevated until January, but concentrations of 17,20βP did not change with season. However, 17,20βP concentrations in spermiated fish were higher than in non-spermiated fish. Daily changes in gonad condition indicated that females undergo daily cycles of ovarian maturation with ovulation occurring after midday. Plasma T and 17,20βP concentrations of females were elevated at midday in association with FOM, but E2 showed no diel change. In males, partially spermiated fish were dominant in the early morning and fully spermiated fish at midday. Plasma T, 11KT and 17,20βP concentrations were low at midnight and reached maximum levels at 0600 hours.

Age, genetic, and nongenetic factors influencing variation in serum sex steroids and zonal volumes of the prostate and benign prostatic hyperplasia in twins.

We have observed that hereditary and environmental factors have a substantial influence on the plasma content of sex steroids in normal male twins and in men of families with prostatic disease. The contribution of genetic and nongenetic familial factors on the variation of plasma sex steroid concentrations and of the volume of zones of the prostate measured by transrectal ultrasound (TRUS) has now been investigated in pairs of male monozygotic (MZ) and dizygotic (DZ) twin pairs between age 25-75 years. Bioelectric impedance permitted quantitation of body fat, lean body mass, and water. Morphometrics and testicular volume were also determined. The intraclass correlation (rI) was > 0.40 for the variation of the total volume (TV), the transition zone (TZ), the peripheral zone (PZ), and the ratio of TZ/PZ in both MZ and DZ twins, and heredity affected 22% of the variation of the PZ and TZ and 30% of the ratio of TZ/PZ. None of the TV and environmental factors influenced the remainder of the variation. Heredity accounted for 25% or more of the variation of dihydrotestosterone (DHT), and the ratios of DHT/testosterone (T), estradiol (E2)/T, androstanediol glucuronide, sex hormone-binding globulin (SHBG)-bound T, T/SHBG, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and dehydroepiandrosterone sulfate (DHEA-S). In MZ twin pairs the variation of the volumes of the prostate became greater with age. In contrast, the variation of the sex hormone concentrations did not show greater variation with age. Heredity also affected > 30% of the variation for waist measurement, weight, body fat, body mass, water and lean body mass, body mass/fat, and testicular size. Our results indicated that both MZ and DZ twin pairs had zonal prostate volumes more like their twin pair than like unrelated twins. However, nongenetic factors exerted stronger influences than genetic factors on zonal volumes of the prostate. In contrast, hereditary factors had stronger influences on determining the variation of many sex hormones and morphometric characteristics than did nongenetic factors.

Sex Differences in the Heart Rate Variability Spectrum of Free-Swimming Atlantic Salmon (Salmo salar L.) during the Spawning Season

Heart rates were telemetered from male and female adult salmon (Salmo salar L.) when they were ascending a river to spawn. In this specific period, males had higher heart rates than females, reflecting the higher activity levels and higher metabolic rates of the males. Analysis of the beat-to-beat intervals (using the heart rate variability spectrum) revealed a single spectral peak that differed between sexes, with males displaying a spectral peak at lower frequencies than females even when the heart rate was the same. The dual spectral peak observed in higher vertebrates and in some other studies in fish was not found. It is suggested that the single spectral peak is related to the activity of the blood pressure control loop. In this framework, the gender spectral differences could be explained by a differential vascular reactivity related to different sex steroid concentrations in plasma. Spectral analysis of beat-to-beat intervals is extensively used to understand short-term control of heart rate in mammals; this study indicates that it can also be applied to free-living fish to understand neural cardiovascular regulation.

Ecoestrogens and embryos—Is there a scientific basis for concern?

Abstract A variety of estrogenic compounds exist in the environment, both natural (phyto- and fungal estrogens) and synthetic (e.g. pesticides, industrial contaminants). Recent studies have suggested that a number of reproductive abnormalities in wildlife and human populations could be due to embryonic exposure to ecoestrogens. These abnormalities include modifications in gonadal structure and functioning, abnormalities in genital morphology and size, depressed plasma sex steroid concentrations and various reproductive organ cancers. We review briefly some of these studies as well as those providing data that identify various ecoestrogens. The data to date suggest that a wide variety of compounds can interact with the estrogen receptor and can stimulate estrogen-associated responses. Compared with 17β-estradiol, ecoestrogens have a weak binding affinity for the estrogen receptor and thus have been described as weak estrogens. However, we hypothesize that the estrogenic action of these compounds is augmented by their bioavailability and persistence. Initial testing demonstrates that some ecoestrogens show relatively little affinity for plasma binding proteins; thus, the majority of the estrogenic compound in the plasma is available for translocation into the cell. Likewise, many synthetic ecoestrogens appear to persist for months or years, stored in body fat. Future research needs to examine in more detail the relative roles of receptor affinity, cellular availability, and nuclear persistence in determining the estrogenicity of ecoestrogens. Only then, will we begin to understand the true role of these compounds in ecosystems.

Seasonal Changes in Plasma Concentrations of Sex Steroids in the Salamander Hynobius nigrescens

Changes in plasma concentrations of androgens, estradiol-17β, and progesterone were examined throughout the year in Hynobius nigrescens. The highest mean concentration of androgens (312.7 ng/ml) was found in males in the process of spermiation and having just entered the breeding pond in early spring. Plasma estradiol- 17β concentration was at its base level in females both before and after the animals had entered the pond. Plasma progesterone concentrations were low throughout the year except during the aquatic phase in March when females possessed some ova within the coelom and many ova in the oviducts and ovisacs after the completion of ovulation. During the terrestrial phase, or nonbreeding season, plasma concentrations of androgens and estradiol-17β were in approximate correspondence with the development of sex accessory structures. This is the first documentation of plasma concentrations of sex steroids in hynobiid salamanders.

Body measurements, estrogen availability and the risk of human breast cancer: a case-control study.

Recent studies on lifestyle-related mechanisms involved in cardiovascular risk offer important clues for a better understanding of breast-cancer epidemiology. Central body-fat distribution promoted by an affluent dietary intake and a sedentary lifestyle over many years is related to elevated serum triglycerides and free fatty acids, with lower levels of sex-hormone-binding globulin (SHBG). The resulting greater availability of estradiol not bound to SHBG could help to explain the high breast-cancer incidence in Western industrialized countries. We conducted a case-control study comparing 225 women aged 38 to 75 years with operable (stage I or II) breast cancer and 441 women of the same age having no breast cancer who participated in a population-based breast-cancer screening program. Body fatness, as measured by body mass index (BMI), fat distribution as measured by waist-to-hip girth ratio (WHR), body height, serum lipids, SHBG and the available fraction of estradiol were analyzed in a conditional logistic regression, together with family history for breast cancer, reproductive history and smoking. Post-menopausal cases showed no difference in body fatness (BMI), but a significant preponderance of central adiposity (WHR). In contrast, pre-menopausal cases were significantly leaner, but had a similar body-fat distribution as compared with controls. In all women, WHR, and less strongly BMI, was positively correlated with serum levels of triglycerides and available estradiol fractions. An independent, positive linear correlation between body height and relative risk (RR) was observed. Moreover, a significant correlation between SHBG and menarcheal age was seen in cases, but not in controls. These data support our hypothesis that lifestyle relates to breast-cancer risk by metabolic-endocrine mechanisms which modulate the availability of individual sex-steroid concentrations in plasma. The findings of height as a risk factor and adult SHBG levels being correlated with menarcheal age suggest that lifestyle factors promoting breast-cancer development already act around puberty. The leanness of pre-menopausal cases awaits further explanation.

Reproductive cycles of male and female tuatara (Sphenodon punctatus) on Stephens Island, New Zealand

Cycles in gonadal activity and plasma sex steroid concentrations were investigated in wild female and male tuatara on Stephens Island, New Zealand. Females nest once every four years on average. Vitellogenesis is spread over the first three years, and mating, ovulation and nesting occur in the fourth. Oviducal eggs are carried for6–8 months before nesting. Although the length of this ovarian cycle is unparalleled among oviparous reptiles, the associated cycles in plasma concentrations of sex steroids are similar to those in other reptiles. Mean plasma concentrations of oestradiol increase during vitellogenesis, peak at mating, fall rapidly before or around ovulation, are low during most of gravidity, and rise slightly during late gravidity‐nesting. Plasma concentrations of testosterone show a similarcycle. Mean plasma concentrations of progesterone are low during vitellogenesis and peak around ovulation. This periovulatory surge falls within1–2 months, and mean concentrations are low during the final 5–6 months of gravidity. Male tuatara show an annual, pre‐nuptial reproductive cycle. Mean plasma concentrations of testosterone are low during winter, rise during spring, peak during midsummer‐early autumn, when pre‐nuptial displays and mating occur, and fall in mid autumn. Limited histological data indicate that spermiogenesis occurs during midsummer‐early autumn, and also support a previous study suggesting that there may be no period of complete testicular and epididymal regression. In comparison with the pre‐nuptial cycles of other temperate‐zone reptiles, the cycle in male tuatara shows a more prolonged duration of testicular activity and of rising or elevated plasma testosterone concentrations. The prolonged reproductive cycles of male and female tuatara may be adaptations to a temperate environment with cool summers and cool, but not freezing, winters.

Identification of female tuatara in ovulatory condition using plasma sex steroid concentrations

Abstract Female tuatara (Sphenodon punctatus) are long-lived reptiles that do not nest each year. Identification of gravid females (those carrying oviducal eggs) has in the past been achieved only with radiography or laparoscopy. Both techniques require considerable equipment and/or operator skill, and both pose an unquantified degree of risk to tuatara. For these reasons, we sought to develop a blood test for identifying females destined to nest in a given year. We observed that on Stephens Island, New Zealand, those females that nested in a given year had significantly higher plasma concentrations of oestradiol, testosterone, and progesterone at the time of mating-ovulation (February-March) than in those females that did not ovulate and nest that year. Used in conjunction, the criteria of a testosterone concentration ≥2.0 ng/ml or a progesterone concentration ≥5.8 ng/ml in plasma, obtained within 3 h of capture in February–March, can be used to distinguish females destined to nest that year from those that do not nest. These criteria may prove useful for comparing nesting frequencies among years on Stephens Island, for comparing nesting frequencies among different islands, and for monitoring reproduction in captivity. To our knowledge, this is the ftrst time that plasma hormone concentrations have been used to identify nesting females in reptiles that do not nest each year.

Plasma sex steroid concentrations in male blue cod, Parapercis colias (Bloch &amp;amp; Schneider) (Pinguipedidae), sampled underwater during the spawning season

Blood samples were collected underwater from male blue cod (Parapercis colias) captured by SCUBA divers during the spawning season. Acute reproductive condition (degree of spermiation) and size were determined, after which the fish were released. In all, 82 fish were sampled during 13 dives in daylight hours over a 10-week period. Fish that were not spermiated were most common before 1000 hours, whereas most of the spermiated fish were captured after 1500 hours. Spawning has yet to be observed, but available evidence suggests that it occurs in the late afternoon or early evening, although it is not known how frequently spawning acts occur. Plasma testosterone (T) concentrations were variable but did not show consistent association with date, time of sampling, moon phase or acute reproductive condition. Plasma T was higher in large males than in small males in all spermiation stages. There was no clear relationship between plasma T and T-glucuronide concentrations. Plasma 11-ketotestosterone (IIKT) concentrations were less variable than T concentrations but, like T, were higher in large fish than in small fish. Plasma 17a,20�P-dihydroxy-4-pregnen-3-one (17,20�P was not detectable at any time, but the presence of 17,20�P-glucuronide (17,20�PG) indicated that 17,20�P was being produced and glucuronated before it accumulated to detectable concentrations in the plasma. In spermiated fish, 17,20�PG was elevated.

Effects of a fat-containing meal on sex hormones in men

The effect of a fat-containing meal on plasma sex steroid concentrations was investigated in normal men. After an overnight fast on two separate occasions, subjects ingested a liquid meal containing either a nonnutritive sweetener (control), or isocaloric meals of mixed calorie sources with either high-fat content or mixed carbohydrate and protein with minimal fat. The order of the meals was alternated. Blood samples were collected at 15-minute intervals and pooled each hour. Sampling began at 7:00 am and the test meal was ingested at 8:00 am. Sex steroids, including estrone, estradiol, testosterone, and dihydrotestosterone (DHT), sex hormone-binding globulin (SHBG) capacity, free testosterone concentration, and luteinizing hormone (LH) were determined by either specific radioimmunoassay or dialysis. The fat-containing meal, but not the nonnutritive or mixed carbohydrate and protein meal, resulted in a significant ( P < .01) reduction in total and free testosterone. Estrogens and luteinizing hormone were unaffected by either meal. This is the first documentation, to our knowledge, of the acute effect of a fat-containing meal on sex steroid concentrations in blood. Our observations suggest that a fat-containing meal reduces testosterone concentrations without affecting luteinizing hormone. This might indicate that fatty acids modulate testosterone production by the testes.

Absence of daily cycles in plasma sex steroids in male and female tuatara ( Sphenodon punctatus), and the effects of acute capture stress on females

The possible existence of daily cycles in plasma concentrations of sex steroids was examined in wild male and female tuatara ( Sphenodon punctatus). Samples were collected from freshly captured animals at dusk, middle of the night, dawn, and middle of the day in January (summer) and July (winter). Males showed daily cycles in mean body temperature ( T b) in both seasons but no daily cycle in mean plasma testosterone concentration in either season. Vitellogenic female tuatara in January and females in mixed reproductive condition in July also showed significant daily variation in T b. However, there were no daily cycles in mean plasma concentrations of estradiol, testosterone, or progesterone in either group of females. Vitellogenic female tuatara subjected to an acute capture stress (3-hr confinement) in January had mean plasma concentrations of estradiol and testosterone that did not differ from those of free-roaming females. However, progesterone and T b were significantly higher in captives than in free-roaming females. The elevation in progesterone may result from physical confinement, the difference in T b, or both. These data suggest that seasonal fluctuations in circulating concentrations of plasma sex steroids in tuatara can be determined using samples collected at different times of the 24-hr cycle. However, the effects of acute capture stress and/or changes in T b on plasma progesterone concentrations need to be considered in future studies on this and possibly other female reptiles.

Fluctuation in sex steroids and sex steroid-binding protein during the development and annual cycle of the male common carp, Cyprinus carpio

1. 1. Sex steroid concentrations increased during the spawning season in the juvenile and adult male common carp. 2. 2. High levels of plasma sex steroids also were found during the non-spawning season in juvenile fish. 3. 3. No correlation between plasma sex steroid concentrations and SBP binding levels was found in the course of development and annual cycle of male carp. 4. 4. The detection of T concentrations in larval carp occurred earlier than that of T-binding level.

Elevation of sex steroids and inhibition of PDP-glucuronyltransferase are out of phase during gonadal maturation in the common carp

1. Plasma sex steroid concentrations, onset of gonadal maturation, and hepatic microsomal UDP-glucuronyltransferase (UDPGT) activities were followed under natural temperature and photoperiod in outdoor tanks, and under controlled laboratory temperature and photoperiod regimens in common carp ( Cyprinus carpio). 2. Decreased activity of UDPGT was out of phase with elevations in plasma testosterone and 17β-estradiol during gonadal maturation. 3. Injection of pituitary extract induced final gonadal maturation and transient elevations (within 24 hr) of both plasma sex steroid concentrations and UDPGT activities. 4. There were no simple relationships between plasma sex steroid concentrations and activity of hepatic microsornal UDPGT in common carp.

Seasonal variations in testicular germ cell stages and in plasma concentrations of sex steroids in male rainbow trout ( Salmo gairdneri) maturing at 2 years old

Male rainbow trout of an autumn- and a spring-spawning strain were sampled every 28 days over a period of 2 years, starting when they were approximately 6 months old. Plasma concentrations of testosterone (T), 11-ketotestosterone (11-K), and 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) were measured by radioimmunoassay. A histometric method was used to determine the absolute quantities of several germ cell stages, including spermatogonia B, spermatocytes, spermatids, spermatozoa, and resorbing spermatozoa. The general pattern of gonadal development in the two strains was similar, e.g., steroids peaking in the order T → 11-K → 17,20β-P. However, there were a large number of differences—in the timing of first appearance of various parameters, in the timing and magnitude of various peaks, in the histological appearance of the regressing testes, and in the heterogeneity of the male population. T and 11-K levels were not associated with any particular germ cell stage in the two strains. Peak 17,20β-P levels coincided with the period when ovulated females were present and milt production was maximal.