The present study correlates the profiles of certain ovarian steroid hormones with stages of oocyte maturation in common carp induced to spawn by classic hypophysation. Sixteen females were injected at midday with a calibrated homologous pituitary extract (cPE) containing 0.07 mg immunoreactive cGTH/kg body wt, as a priming dose. Ten of those females were injected 11 hr later with cPE containing 0.35 mg IR cGTH/kg body wt as an induction dose (Group P + IN). The other females were left without further treatment (Group P). An additional 6 fish were injected around midnight with the induction dose only (Group IN). A control group was injected with 0.7% saline (0.2 ml/kg) at midday and at midnight (Group S). Ovarian biopsies were taken at intervals and the maturational stages of individual females were recorded. Blood was sampled simultaneously with the biopsies. GTH, estradiol (E2) and 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P) were determined by specific RIAs. Germinal vesicle breakdown (GVBD) and ovulation were observed only in Group P + IN 2-4 hr after the second injection. The change in circulating ovarian steroids, in response to hypophysation, was found to depend on the maturational stage of the oocytes. Fish with oocytes showing central and eccentric GV responded to the increased GTH level by elevating E2, but not 17,20-P in the circulation. However, fish with ovaries containing peripheral GV responded by an increase in circulating 17,20-P (111 ± 14.6 ng/ml) concomitantly with a transient decrease in E2. Only fish that showed such a shift in the steroidogenic profile exhibited GVBD in their oocytes and consequently ovulated. This finding is consonant with the hypothesis that 17.20-P is the maturational-inducing steroid in the carp.