One-year-old male rainbow trout (Oncorhynchus mykiss) of the Comec Autumnal strain, reared in natural conditions of temperature (February: 8.5°C; August: 20°C) and photoperiod (48°N) at the INRA experimental fish farm (Sizun, Finistere, France), were used. During the entire experimental period (January-November 1995), fish were kept in circulating fresh-water tanks (capacity 1800 L) and fed once daily (6 days/wk, except for 2 days prior to sampling) with commercial pellets (Aqualife no. 17; Aqualim S.A, St. Es-tephe, France). birth control pills
In order to describe the changes in GH-R in testes during one spermatogenetic cycle, 15-25 males (sexually immature, 505 ± 32 g body weight in January; sexually mature, 2260 ± 92 g body weight in November), randomly caught, were killed at approximately 30-day intervals (n = 11 samplings). At each sampling, after rapid anesthesia (3-4 min) in phenoxy-2-ethanol (0.5:1000 v:v), the body weight (± 1 g) and length (± 0.1 cm) of each male were measured. Blood samples were collected rapidly from the caudal vasculature in heparinized syringes. The samples were then centrifuged (4°C) at 3200 X g for 20 min, and the plasma was stored in aliquots at -20°C until assayed. Testis and liver were dissected out, weighed in order to determine go-nadosomatic index (GSI = testicular weight X 100/body weight) and hepatosomatic index (HSI = hepatic weight X 100/body weight), aliquoted individually, frozen in liquid nitrogen, and stored at -70°C until used.