17β-oestradiol, testosterone, 11 -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 17β-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 211 ng ml-1 in November and 115 ng ml-1 in January. Ovulation occurred from December to February.