Concentrations of plasma sex steroids, cortisol, and thyroxine were measured by radioimmunoassay in hatchery coho salmon (Oncorhynchus kisutch during winter and early spring. Mean plasma 11-ketotestosterone (11-KT) and estradiol levels fell into two distinct categories: 11-KT was 181-373% higher in males than in females, and estradiol was 109-143% higher in females than in males. No changes in plasma levels of estradiol in fish of both sexes, or in levels of 11-KT in males, were evident during spring when plasma thyroxine and cortisol were markedly elevated, indicating that the fish were undergoing smoltification. Although plasma 11-KT in females appeared to be lower in late April than in February, it showed no correlation with plasma thyroxine or cortisol in these individuals. Our finding of sexual dimorphism in 17 alpha-20 beta-dihydroxy-4-pregnen-3-one was inconsistent between stocks of fish and among sampling dates, thus making interpretation of the results difficult. However, no relationship between this steroid and plasma thyroxine or cortisol was observed. Therefore, plasma levels of sex steroids do not seem to be related to the changes in plasma thyroxine or cortisol observed during smoltification of coho salmon.