The plasma concentrations of estradiol-17 alpha (E2), 17 mu-hydroxy-20 beta-dihydroprogesterone (DHP), progesterone, 11-ketotestosterone (11-KT), testosterone (T), and gonadotropin (GtH) were measured in adult coho salmon (Oncorhynchus kisutch) concomitantly with advancing maturation of the gonads during the spawning season. Concentrations of E2 were higher in females with eggs showing central (premigrating) germinal vesicles, migrating germinal vesicles, or peripheral germinal vesicles than in fish in which the eggs had undergone germinal vesicle breakdown or ovulation. Conversely, plasma levels of DHP were low at central, migrating, and peripheral germinal vesicle stages and then increased dramatically at germinal vesicle breakdown and ovulation. Levels of 11-KT, T, and GtH generally increased with advancing maturity of the eggs. When time rather than degree of maturation was used as a variable, plasma E2 was high in females first returning from the ocean but then dropped off precipitously in fish sampled at the end of the run. Plasma DHP followed the opposite pattern, being low in fish sampled early in the run and high in fish sampled late in the run. No easily discernible patterns emerged from the profiles of the other steroids and GtH, although some significant variation in the concentrations occurred. Even though sampling was initiated at the time when the salmon first returned from the ocean, most of the hormones were not variable in males when viewed according to sampling date or predominant cell type in the testes, which indicates that the males were extremely close to final maturity during the sampling period. Plasma levels of DHP were higher in males that were producing milt than in any other group of males. Concentrations of 11-KT were higher than those of T in all males, but no patterns in the levels of either steroid emerged.