17P-oestradiol, testosterone, 1 1 -ketotestosterone and calcium were measured in plasma of female rainbow trout over the course of a single spawning season. The patterns of rise and fall of the levels of 17/3-oestradiol and calcium during sexual maturation were similar to those demonstrated by other workers. Very high levels of testosterone were found in plasma of sexually mature fish-mean level 2 1 1 ng ml-1 in November and 1 I5 ng ml-' in January. Ovulation occurred from December to February.

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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Plasma growth hormone concentrations were determined in diploid and triploid rainbow trout of both sexes during sexual maturation. Diploid females grow large ovaries, whereas triploid female trout show no ovarian development. The plasma growth hormone concentration in triploid female trout remained low and unchanged throughout the study, whereas it rose slightly, but significantly, in the diploid females that matured, but not in those that remained immature. On the other hand, triploid males do develop testes like their diploid counterparts. In both groups spermiation was accompanied by a steady rise in the plasma growth hormone concentration. The results suggest that the elevated growth hormone concentration in mature male trout was a consequence not of reproduction per se, but of the loss of condition that accompanied spawning. This hypothesis was supported by the results from the females. Apart from a temporary loss in the mature diploids caused by stripping of the eggs, female trout did not lose condition, neither did they show any change in the plasma growth hormone concentration during the period when they ovulated. These results suggest that it was the nutritional insuffrciency accompanying reproduction in male fish that caused the elevation in growth hormone concentration. In fact, a strong negative correlation between the plasma growth hormone concentration and the condition factor of the fish was observed. Q

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