Plasma levels of C18-, C19- and C21-steroids in captive and feral female sea bass

Morphometric analysis of the gonads of sea bass Dicentrarchus labrax revealed that captive fish matured 1 month later than feral fish, but levels of gonadal steroids were identical in both groups at the same stage of sexual development. 17β-oestradiol (E₂) (up to 3 ng ml-1) and testosterone (T) (up to 4 ng ml-1) were highest during the gametogenetic period while 17,20β,21-trihydroxy-4-pregnen-3-one (17,20β,21-P) (free and sulphated) were maximal during the spawning period. Free 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) was very low and did not change (c. 0·5 ng ml⁻¹) while 17,20β-P-sulphate increased during the spawning period in both groups (up to 2 ng ml⁻¹). In contrast cortisol levels were higher in captive fish and increased during the spawning period (up to 100 ng ml⁻¹). These results suggest that captivity delays vitellogenesis and spawning in sea bass without affecting the final levels of the gonadal steroids and further indicates a role for cortisol in the latter period. The increased levels during the spawning period suggests a pheromonal role for 17,20β-P-sulphate and 17,20β,21-P-conjugates and the involvement of 17,20β,21-P in final ooccyte maturation.     

Stress-induced changes in concentrations of plasma sex steroids in black bream

Cortisol levels of black bream Acanthopagrus butcheri at capture did not change with time of day, gonadal stage or season and were 1·9±0·2 and 2·8±0·4 ng ml⁻¹ for male and female fish, respectively. Confinement resulted in significantly elevated cortisol levels at all time periods; however, levels after 24 h of confinement were significantly lower than peak cortisol levels (15 min for males and 1 h for females). Confinement stress resulted in reduced levels of 17β-oestradiol (E2) and testosterone (T) within 1 h in sexually mature females. In mature males, suppression of T and 11-ketotestosterone (11KT) occurred after 30 min and 6 h of confinement, respectively. The relationship between confinement stress and levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20β P) was more complex, with levels in males being elevated after 15 min and 24 h and suppressed after 6 h of confinement. In contrast, 17, 20β P levels in females were elevated after 1 h of confinement. In regressed females, plasma E₂ and T concentrations were low at capture and were not affected by confinement stress whereas plasma 17, 20β P was elevated within 1 h. This study indicates that stress exerts a rapid inhibitory effect on gonadal steroidogenesis.     

Variation in plasma oestrogens and androgens during the seasonal and semilunar spawning cycles of female gulf killifish, Fundulus grandis (Baird and Girard)

Variations in 17β-oestradiol, oestrone, testosterone, and 11-ketotestosterone were measured by radioimmunoassay in the plasma of female Gulf killifish, Fundulus grandis , during their seasonal and semilunar spawning cycles. Both 17β-oestradiol and testosterone exhibited distinct seasonal variation, peaking very early in the breeding season during March and April, decreasing gradually thereafter the cessation of spawning in late August and the seasonal regression of ovaries in September, and eventually falling below the detectable limits of our assays during the very early stages of seasonal ovarian recrudescence in November. Both steroids also exhibited distinct semilunar variation within the breeding season, with highest plasma concentrations immediately prior to, and during, each spring tide spawning. Such results suggest that these steroids have physiological roles in the generation and regulation of both seasonal and semilunar reproductive cycles: 17β-oestradiol by controlling development of vitellogenic oocytes; testosterone perhaps by acting as a precursor in the production of oestrogens and other steroids. In contrast, oestrone and 11-ketotestosterone were only rarely detected, implying that these particular oestrogens and androgens are probably not physiologically active in female killifish.     

Plasma cortisol and 17β-oestradiol levels in roach exposed to acute and chronic stress

Plasma cortisol levels were measured as an indicator of physiological stress in roach subjected to brief handling, or to a 14-day period of confinement, and in undisturbed control fish, during winter (water temperature 5° C) and summer (16° C), at which time plasma 17 β-oestradiol levels were also determined. Cortisol levels in undisturbed roach were low (mean 8·1 ng ml⁻¹ at 5° C; 1·4 ng ml⁻¹ at 16° C) and both handling and handling+confinement elevated blood cortisol levels significantly to 400 and 140 ng ml⁻¹, respectively (at 5° C) and 700 and 600 ng ml⁻¹, respectively (at 16) C). Blood cortisol levels had almost returned to baseline within 4 h following handling alone but in fish subjected to handling and prolonged confinement cortisol levels remained elevated for up to 168 h. Differences in baseline and poststress levels of cortisol, and in the rate of recovery from acute stress, were observed at the two different temperatures and the possible factors underlying these differences are discussed. Circulating levels of 17 β-oestradiol were reduced significantly within 24 h of exposure to either acute handling or chronic confinement indicating that the reproductive endocrine system in roach is sensitive to disruption by stressors.     

Rapid bioconcentration of steroids in the plasma of three-spined stickleback Gasterosteus aculeatus exposed to waterborne testosterone and 17β-oestradiol

The relationship over time between the concentrations of two steroids, singly and in combination, in a static exposure system and in the blood of three-spined stickleback Gasterosteus aculeatus , held within the exposure system was investigated. Groups of three-spined stickleback were exposed (nominally) to either 1000 ng l⁻¹ 17β-oestradiol (E2), testosterone (T) or E2 and T in combination at the same concentrations for 6 days. Both water and fish were sampled at intervals and steroid concentrations in both compartments were determined. The plasma steroid time profile revealed a rapid bioconcentration within the first 6 h of exposure. The plasma steroid levels attained at this time point (20–90 ng ml⁻¹) were up to 50-fold (E2) and 200-fold (T) greater than the actual levels of steroid measured in the exposure water, while levels in the blood of control fish did not exceed 4 ng ml⁻¹. The substantial elevation of plasma steroid levels relative to the concentrations of steroid to which the fish were exposed in the ambient water gives scope for delivery of the steroids to target endocrine tissues at levels far in excess of what might be predicted on the basis of passive branchial uptake alone. These results are discussed in relation to endocrine disruption, and in particular the occurrence of effects in fish exposed to levels of endocrine active substances that are seemingly physiologically irrelevant.     

Hormonal regulation of the European sea bass reproductive cycle: an individualized female approach

Fifteen tagged female sea bass Dicentrarchus labrax were sampled weekly from September to April and plasma vitellogenin (VTG), testosterone (T), 17β-estradiol (E₂), and two potential maturation inducing steroids (MISs): 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) assayed. An oocyte sample was obtained via intraovarian cannulation at each sampling time from every female and the stage of development of the most advanced clutch of oocytes determined and related to VTG and hormone plasma levels for each female. The mean number of ovulations per female was 1·75+0·25 when those females that did not present ovulations were excluded and up to 4 ovulations detected in some females. The highest plasma levels of T ( c. 6 ng ml^-1) were observed during postvitellogenesis and the beginning of maturation while maximum plasma levels of E2 (>5 ng ml^-1) were obtained during late vitellogenesis. VTG plasma levels increased throughout vitellogenesis peaking ( c. 2·5 mg ml^-1) at postvittelogenesis. For the first time significant changes of plasma progestogens were detected in European sea bass during the sexual cycle. The highest plasma level of 17,20βP ( c. 1·1 ng ml^-1) was observed during postvitellogenesis while the highest level of 20βS ( c. 1·4 ng ml^-1) coincided with final maturation. These results suggest that 17,20βP and 20βS play a role in the early and final maturation, respectively, in the European sea bass.     

Effects of constant short and long photoperiod regimes on the spawning performance and sex steroid levels of female and male sea bass

Under constant short photoperiod, the spawning time of 2-year-old sea bass Dicentrarchus labrax was advanced as compared to controls, whereas spawnings were delayed under constant long photoperiod. High plasma levels of 17β-oestradiol (E_2/) and testosterone (T) in females were coincident with the appearance of vitellogenic oocytes in the ovary, while high levels of 11-ketotestosterone (11-KT) and T in males were coincident with the presence of spermiating males. Although plasma levels of E₂ in females and 11-KT in males were low during the remainder of the cycle, levels of T were always >1 ng ml⁻¹ in both sexes, suggesting that T could play an important role during the initial stages of gonadal development. The profiles of E₂ and T in females and 11-KT and T in males exposed to constant short days were similar to those in the control group, but fish which were maintained under constant long photoperiods showed a bimodal pattern of these steroids. The results obtained from fish exposed to constant photoperiod regimes provide further evidence that an endogenous process could be operating to control the reproduction of sea bass.     

Effects of capture and recovery on plasma levels of cortisol, lactate and gonadal steroids in a natural population of rainbow trout

Rainbow trout were captured by angling from a run of spawning fish on the Tongariro River in northern New Zealand, to examine the effects of catch and release angling on stress and reproductive parameters. Fish were blood sampled immediately after capture at playing times of <5 or 15 min, or after 1 or 24 h of recovery in stream enclosures. Plasma samples were assayed for cortisol (F), lactate, testosterone (T), 17β-oestradiol (E₂), and 17a,20β-dihydroxy-4-pregnen-3-one (17,20βP). Plasma F levels were similar to those of hatchery stocks of rainbow trout, at capture, and became significantly elevated 1 h after capture. Plasma F was still clevated in some fish 24 h after capture. Plasma lactate levels began to increase 15 min after capture, were further elevated 1 h after capture, and had returned to normal 24 h after capture. We proposed that metabolic recovery had occurred but that some animals were still experiencing some degree of stress, possibly in response to holding conditions in the river. Both plasma T and E₂ were depressed 24 h after capture, whereas there was no change in plasma 17,20βP. This is consistent with other findings showing that acute stress is associated with depression of plasma levels of T and E₂. There was no mortality as a result of capture or any of the handling protocols. We conclude that catch and release angling will result in negligible mortality, but may have an inhibitory effect on some reproductive processes.     

Exercise-induced hormone responses in girls at different stages of sexual maturation

The dependence of exercise-induced hormone responses on sexual maturation was tested in a 3-year longitudinal experiment on 34 girls (aged 11–12 years at the beginning). Sexual maturation was evaluated by Tanners five-stage scale. Children cycled for 20-min at 60% maximal oxygen uptake once a year. Cortisol, insulin, growth hormone, β-oestradiol, progesterone and testosterone concentrations in venous blood were determined by radioimmunoassay procedures. Basal concentrations of growth hormone increased and of cortisol decreased when breast stage III was reached. Reaching breast stage IV was associated with an increase in basal concentrations of β-oestradiol, progesterone and testosterone. The exercise induced significant increases in concentrations of cortisol, growth hormone and β-oestradiol and a decrease in insulin concentration. At breast stage III the increase in cortisol concentration was to a lower level [467 (SEM 42) vs 567 (SEM 46)nmol · l⁻¹] and growth hormone concentration to a higher level [29.4 (SEM 0.5) vs 12.8 (SEM 0.4)ng · ml⁻¹], while the fall in insulin concentration was less pronounced [postexercise level 10.6 (SEM 0.9) vs 7.8 (SEM 0.8)mU · l⁻¹] than in stage II. The magnitude of the cortisol response was reduced in the last stage of breast development (+42.1% vs +55.5% at stage II, +66.2% at stage III, and +50.0% at stage IV). The magnitude of β-oestradiol response was the lowest in breast stage IV (+15.8%) and the highest at stage V (+41.1%). The progesterone response became significant at stage IV and testosterone response at stage V. In conclusion, we found that reaching breast stage III was associated with altered responses of cortisol, insulin and growth hormone concentrations while the responses of the sex hormone concentrations became pronounced in the last stages of sexual maturation. Accepted: 17 September 1997     

Plasma non-esterified fatty acid profiles and 17β-oestradiol levels of juvenile immature and maturing adult American eels in the St Lawrence River

American eels Anguilla rostrata , collected from three distinct locations along the St Lawrence River (Lake St Lawrence, Quebec City and Kamouraska), were thought to consist entirely of out-migrating adults at Quebec City and Kamouraska and of both resident and migrants at Lake St Lawrence. The mean plasma levels of 17β-oestradiol closely paralleled the trend observed with gonadosomatic index (I_G) and oocyte development. The highest levels of 17βoestradiol and I_G were observed in Quebec City and Kamouraska, while the levels observed in the resident eels from Lake St Lawrence were very low. Generally, the concentration of total plasma non-esterified fatty acids (NEFA) appeared to follow the trend of 17β-oestradiol and I_G and increased with sexual maturity. However, the most sexually mature group, eels from Kamouraska, had the lowest total plasma NEFA levels. The reason for the drop is not clear at present. In addition, a number of shifts in the plasma NEFA profiles of American eels did occur during their migration in a downstream direction, with several fatty acids either increasing or decreasing in relative abundance. The ratio of n3: n6 fatty acids declined with increasing sexual maturity as most n3 fatty acids became less abundant. Although the correlation between gonadal development and the relative concentration of individual fatty acids was not always clear, both arachidonic acid (20: 4n6) and docosahexaenoic acid (22: 6n3) were significantly higher in the plasma of out-migrating adults than in resident yellow phase eels.     

Plasma concentrations of ovarian steroids in relation to oocyte final maturation and ovulation in female plaice sampled at sea

Blood plasma concentrations of free 17 β -oestradiol, free testosterone and glucuronidated testosterone were strongly positively related to the percentage of vitellogenic oocytes remaining in the ovaries of plaice Pleuronectes platessa caught at sea–being at their highest in pre-spawning (stage IV) females (i.e. those in which the oocytes were close to fully grown, but had not yet entered the stage of final maturation). In contrast, the concentrations of free and sulphated 17,20 β -P, 3α aL ,17, 20 β -P-5 β , and 3 α ,17,21-P-5 β were at their lowest in stage IV females. Free 17,20 β -P (the putative maturation-inducing steroid) became only slightly elevated (less than twofold) during spawning (i.e. in stage V and VI females with hydrated and/or ovulated eggs). Sulphated 17,20 β -P and 3 α ,17,21-P-5 β became slightly more elevated (three- to fourfold). However, sulphated 3 α , 17,20 β -P-5 β concentrations increased 30-fold and were at their highest in fish in which only 40% of vitellogenic oocytes remained in the ovaries. Sulphated 17,20 β -P, 3 α , 17, 20 β -P-5 β and 3 α ,17,21-P-5 β concentrations were significantly positively related to hyaline oocyte batch size; and sulphated 17,20 β -P and sulphated 3 α , 17,20 β -P-5 β were significantly negatively related to the degree of hydration of the hyaline oocytes. None of the steroid concentrations, however, was related to the time of capture. More ovulated females were found in the afternoon than at any other time of the day.     

Analysis of the reproductive cycle of female wild marble trout <Emphasis Type="Italic">Salmo marmoratus</Emphasis> in a prealpine brook of the Soča River basin (Northeast Italy)

The reproductive cycle of female wild marble trout Salmo marmoratus was studied in a prealpine watercourse (Northeast Italy). Gonadosomatic index, oocyte growth and plasma levels of 17β-oestradiol and testosterone were bimonthly measured during one year to obtain a detailed dataset and to gain useful tools for the identification of the reproductive stage without impact for the species. Monitored features showed significant variations during the study period: gonadosomatic index and oocyte size increased slightly but significantly during the first part of the cycle (from February to June), while steroid levels remained quite constant; for all parameters, major increases were highlighted between August and the spawning season (which occurred from the middle of November to the middle of December). Ranges and trends observed for gonadosomatic index (from 0.87 ± 0.41% to 10.91 ± 3.37%) and oocyte diameter (from 0.692 ± 0.031 mm to 4.624 ± 0.208 mm) were generally in agreement with literature regarding salmonids, while plasma levels of 17β-oestradiol (from 0.214 ± 0.015 ng ml^?1 to 78.090 ± 23.882 ng ml^?1) and testosterone (from 0.327 ± 0.086 ng ml^?1 to 71.800 ± 29.406 ng ml^?1) showed wider ranges. A strong non-linear relationship was found between oocyte size and 17β-oestradiol plasma concentration (r ² = 0.890) and especially between oocyte size and testosterone plasma concentration (r ² = 0.947). This last relationship could likely be used to obtain information relative to the gonadal development, especially in the case of an endangered species like Salmo marmoratus, which needs non-invasive tools for management.     

Sex hormone modulation of cell growth and apoptosis of the human monocytic/macrophage cell line

Sex hormones seem to modulate the immune/inflammatory responses by different mechanisms in female and male rheumatoid arthritis patients. The effects of 17β-oestradiol and of testosterone were tested on the cultured human monocytic/macrophage cell line (THP-1) activated with IFN-γ in order to investigate their role in cell proliferation and apoptosis. Activated human THP-1 cells were cultured in the presence of 17β-oestradiol and testosterone (final concentration, 10 nM). The evaluation of markers of cell proliferation included the NF-κB DNA-binding assay, the NF-κB inhibition complex, the proliferating cell nuclear antigen expression and the methyl-tetrazolium salt test. Apoptosis was detected by the annexin V-propidium assay and by the cleaved poly-ADP ribose polymerase expression. Specific methods included flow analysis cytometry scatter analysis, immunocytochemistry and western blot analysis. Cell growth inhibition and increased apoptosis were observed in testosterone-treated THP-1 cells. Increased poly-ADP ribose polymerase-cleaved expression and decreased proliferating cell nuclear antigen expression, as well as an increase of IκB-α and a decrease of the IκB-α phosphorylated form (ser 32), were found in testosterone-treated THP-1 cells. However, the NF-κB DNA binding was found increased in 17β-oestradiol-treated THP-1 cells. The treatment with staurosporine (enhancer of apoptosis) induced decreased NF-κB DNA binding in all conditions, but particularly in testosterone-treated THP-1 cells. Treatment of THP-1 by sex hormones was found to influence cell proliferation and apoptosis. Androgens were found to increase the apoptosis, and oestrogens showed a protective trend on cell death – both acting as modulators of the NF-κB complex.     

A pharmacological dose of 17 β-oestradiol produces minimal metabolic stimulation but evokes hepatic necrosis in the immature female serranid Epinephelus akaara

A pharmacological dose of 17β-oestradiol (5 mg kg⁻¹ per injection for a total of five injections over a period of 9 days) increased liver and serum lipid concentration and induced vitellogenin into the serum of the immature female red grouper Epinephelus akaara . The levels of electrolytes, glucose, lactate and protein in the serum, and the protein, glycogen and water content of liver and muscle were not affected. The activities of enzymes regulating gluconeogenesis, glycolysis, Krebs cycle, transamination and glycogen synthesis remained unaltered. These findings are in marked contrast to those of Ng et al . (1984) which showed a strong stimulatory effect of a ten-times smaller dose of 17β-oestradiol on these biochemical parameters. Ultrastructural examination of hepatocytes of these heavily oestrogenized fish revealed that some cells possessed organelles with an abnormally activated appearance while others had undergone necrotic changes, suggesting that some hepatocytes were overstimulated and subsequently degenerated. Results of the present investigation caution against the use of a pharmacological dose of 17β-oestradiol in fish culture.     

Study of the sexual maturity of female bluefin tuna: purification and partial characterization of vitellogenin and its use in an enzyme-linked immunosorbent assay

Plasma steroid levels of female bluefin tuna (BFT) Thunnus thynnus rose from c. 1·5 ng ml⁻¹ during the quiescent period (March) to c. 7 ng ml⁻¹ during the ripening period (May). Testosterone (T) increased further to c. 8 ng ml⁻¹ during the pre-spawning period (June) while 17β-oestradiol (E₂) began to decrease. In the post-spawning period (August) steroid levels decreased to < 1 ng ml⁻¹. Vitellogenin (Vtg) plasma levels seemed to follow changes in E₂, showing an increase from the quiescent period to the ripening period of c. 18 mg ml⁻¹, decreasing slightly before spawning, and then decreasing after spawning. The Vtg content in plasma showed a good correlation both with the plasma levels of E₂ and T and with the percentage of vitellogenic oocytes at different periods of the reproductive cycle. Thus the ELISA could be taken as validated. Immunohistochemical staining of ovaries with anti BFT-Vtg serum demonstrated a high cross-reactivity with yolk proteins allowing the identification of vitellogenic oocytes.     

Dose-related effects of 17βoestradiol (E2) on liver weight, plasma E2, protein, calcium and thyroid hormone levels, and measurement of the binding of thyroid hormones to vitellogenin in rainbow trout, Salmo gairdneri Richardson

Administration of 17β-oestradiol (E₂) to rainbow trout, in the form of hydrogenated coconut oil implants produced a stable, long-term elevation in plasma E₂ levels. The elevation was doserelated (over the range 1–10mg kg-¹ body weight) both 4 and 8 weeks after implantation. Dose-related increases were also observed with respect to liver weight-body weight ratios and plasma protein levels. Plasma T₃ and total calcium levels were depressed and elevated, respectively, by E₂ treatment but the responses were not linearly related to the dose of E₂ administered; there was no significant effect of E₂ on plasma T₄ levels.
E₂ induced a shift in the binding of T₃ to plasma proteins, with T₃ binding to smaller molecular weight proteins; neither T₄ nor T₃ bound to vitellogenin which was present at high levels in the plasma of E₂-treated fish.     

Relationship of plasma steroids to germ cell development and the presence of protamine mRNA in rainbow trout during the induction of spermatogenesis with partially purified salmon gonadotropin

The gonadosomatic index (GSI) of pre–pubertal male rainbow trout, which had been injected biweekly with partially purified salmon gonadotropin (sG–G100, 50 μ mUg kg⁻¹ body weight), increased from 0.05 to 1.85 over 21 weeks from injection, while control GSI remained below 0.05. Plasma testosterone (T) increased from 2 to 11.34 ng ml⁻¹ by week 21 in injected fish, while control level remained below 1.5 ng ml⁻¹. In injected fish plasma 11–ketotestosterone (KT) and 17,20–dihydroxyprogesterone (17α20βP) levels increased from 20.2 to 41.9 and 8.9 to 219.7 ng ml⁻¹ respectively. Plasma T, 11–KT, and 17α20βP were all correlated with the GSI ( P <0–001) in injected fish. The most advanced stage of germ cells present in the control fish were spermatogonia. However, in injected fish spermatozoa were present by week 21. Eggs fertilized at this time with spermatozoa from injected fish achieved a 78% fertilization rate, whereas the testicular homogenate was incapable of fertilizing eggs.     

Urine of reproductively mature female rainbow trout, Oncorhynchus mykiss (Walbaum), contains a priming pheromone which enhances plasma levels of sex steroids and gonadotrophin II in males

Urine of reproductively mature female rainbow trout was shown to contain a priming pheromone which raised the levels of 17 a ,20β-dihydroxy-4-pregnen-3-one (17,20β-P), testosterone and gonadotrophin II in the blood plasma of reproductively mature male rainbow trout. Milt volumes, however, were unaffected.
Because it had been established in a previous study that the sulphated form of 17,20β-P is abundant in the urine of spawning rainbow trout, synthetic 17 a ,20β-dihydroxy-4-pregnen-3-one 20-sulphate was also tested for pheromonal activity. It was found to have only a smalt and inconsistent priming effect on steroid levels and did not alter the orientation or spawning activity of males.
It was also shown that, in the majority of experiments, there was a significant drop, over the course of 24 h, in the levels of 17,20β-P and testosterone in the control groups of males.     

Gonadal morphology, enzyme histochemistry and plasma steroid levels during the annual reproductive cycle of male and female brown bullhead catfish, Ictalurus nebulosus Lesueur

The annual reproductive cycle of the brown bullhead catfish, Ictalurus nebulosus Lesueur, was investigated over a two-year period. In females, GSI increased in the spring as follicles enlarged and the granulosa became hypertrophied, dropped during spawning in August, then rose in the autumn as follicles enlarged slightly. 3β-Hydroxysteroid dehydrogenase (3β-HSD) activity was limited to thecal nests of large, vitellogenic follicles. Plasma testosterone and estradiol-17β levels increased in parallel with GSI. Levels of both steroids dropped prior to the spawning period, although a peak in estradiol-17β was evident during the spawning period. No 11-ketotestosterone was detected in female plasma. In males, GSI increased in the spring as spermatogenesis proceeded, and dropped during spawning. 3β-HSD activity was confined to Leydig cells and was most intense prior to spawning. Plasma testosterone and 11-ketotestosterone peaked during the pre-spawning period, dropped prior to spawning, then rose slowly during the autumn. A peak in estradiol-17β occurred during the spawning period. Significant differences in GSI and plasma steroid levels during the pre-spawning and spawning periods were observed between the two yearly cycles; they may be related to differences in rainfall during these periods.     

Contrasting seasonal modulation of the stress response in male and female rainbow trout

Between June and September the magnitude of the plasma cortisol response of maturing male and female rainbow trout to confinement was indistinguishable. A progressive increase in confinement-induced cortisol levels in both sexes occurred during May to September, associated with the seasonal rise in water temperature. Between September and January a reduction of >50% in the magnitude of the cortisol response to confinement in male fish (but no decline in females) coincided with declining water temperature and significant increases in plasma 11-ketotestosterone and elevated plasma testosterone levels. Plasma oestradiol-17β levels were significantly greater in females than males throughout the study period and this difference was maximal between September and January. However, plasma testosterone was also elevated in females during this period and levels overall were higher than those in male fish. Previous studies have shown oestradiol-17β and testosterone to have diametrically opposed effects on stress responsiveness in trout, with the former enhancing, and the latter suppressing, the cortisol response to a stressor. The relative roles of androgens, estrogen and water temperature in modulating the stress responsiveness of rainbow trout are discussed.