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Measurements were made of plasma 11‐ketotestosterone and testosterone in control and sex‐reversed male rainbow trout, before and during their first spawning season (winter 1978/1979). Plasma concentrations of both androgens (in both groups) were 2–3 ng ml‐1 in January 1978 (apart from some precocious spawners), rose slowly to 9–11 ng ml‐1 in April and July and then increased rapidly to 100–150 ng ml‐1 in November. From this time, testosterone levels declined but those of 11‐ketotestosterone continued to rise to a peak of 260 ng ml‐1 in February 1979. No significant differences in hormone levels were found between control male and masculinized female fish.

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