Previous studies have shown that blood plasma levels of 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP) and 17alpha, 20beta, 21-trihydroxy-4-pregnen-3-one (20beta-S) increase in striped bass (Morone saxatilis) undergoing final oocyte maturation (FOM). Both hormones are produced by ovarian fragments undergoing hCG-induced germinal vesicle breakdown (GVBD) in vitro. In the present study, we investigated binding of DHP and 20beta-S to ovarian membranes from striped bass undergoing FOM. Saturable binding sites for DHP were not detected. Saturation of 20beta-S binding sites with 5 nM [3H]20beta-S occurred within 40 min at 0 degrees C (at 3 min, half of the maximum specific binding of steroid was calculated to have occurred), and the binding was pH-dependent. Scatchard analyses revealed the presence of a single class of high-affinity (dissociation constant [Kd] = 1.4 +/- 0.2 nM), limited-capacity (estimated concentration [Bmax] = 2.7 +/- 0.3 pmol/g ovary) 20beta-S binding sites on membranes from striped bass ovaries undergoing FOM. In contrast, only low levels of specific binding (Bmax < 0.04 pmol/g tissue) were detected on membranes from testes, liver, brain, and muscle. Ovarian membranes prepared from vitellogenic females also had low levels (Bmax < 0.1 pmol/g ovary) of specific 20beta-S binding, less than 5% of that found during FOM. Results of competition assays showed that DHP was approximately 250 times less effective than 20beta-S for displacing 20beta-S from ovarian membranes. In contrast, 20beta, 21-dihydroxy-4-pregnen-3-one was a very effective competitor, although it is only a weak inducer of oocyte GVBD in vitro. Of several other steroids tested, only progesterone showed affinity for the 20beta-S binding site within a physiological range of concentrations. Taken together with previous studies of striped bass FOM, these findings indicate that 20beta-S is the oocyte maturation-inducing steroid hormone in striped bass.