Abstract 17β-Estradiol, testosterone, and 17α,20β dihydroxy-4-pregnen-3-one (17α20βP) levels were measured in plasma samples obtained from coho salmon ( Oncorhynchus kisutch) during the preovulatory period and following the injection of mammalian gonadotropin releasing hormones. Spontaneous reproductive activity was characterized by a rapid decline in plasma 17β-estradiol 10 days prior to ovulation and a large increase in plasma 17α20βP 6 days before ovulation. Testosterone levels remained high (>125 ng/ml) throughout the preovulatory period, with a small peak evident 6 days prior to ovulation. Oocyte development was not accelerated in fish injected with mammalian LH-RH, whereas des-Gly 10[D-Ala 6]LH-RH-ethylamide (LH-RHA DAla 6) promoted germinal vesicle breakdown (GVBD) in 10 out of 14 fish within 96 hr. Only LH-RHA DAla 6-injected fish which completed GVBD displayed the characteristic steroid changes observed during spontaneous reproductive activity. In LH-RH-injected fish, there was a transient increase in plasma 17α20βP levels which persisted for less than 24 hr. LH-RHA DAla 6-injected fish which failed to complete GVBD maintained high 17α20βP levels, but the peak concentrations were lower than those in fish which completed GVBD. These fish also maintained high plasma 17β-estradiol levels when compared to fish which completed GVBD. The appearance of high plasma 17α20βP levels during spontaneous and LH-RHA DAla 6-induced reproductive activity was coincident with the time of GVBD. This finding was consistent with the view that 17α20βP functions as the maturation-inducing steroid in salmonids. The induction of GVBD using gonadotropin-releasing hormones was related to the elevation of plasma gonadotropin levels for greater than 24 hr [ G. Van Der Kraak, H. R. Lin, E. M. Donaldson, H. M. Dye, and G. A. Hunter (1983) Gen. Comp. Endocrinol. 49, 470–476] and a decrease in 17β-estradiol production.