The effect of inhibin on oocyte maturation division was studied in vitro. Inhibin inhibited spontaneous division in both cumulus enclosed and denuded rate oocytes obtained from immature rats. The inhibition was dose-dependent (15-5000 units/ml) with an ED50 of 45.8 units/ml (4.00 nM). Follicle stimulating hormone (FSH) suppressing protein (FSP) which has inhibin-like activity accelerated the rate of meiotic maturation of rat oocytes in culture (P less than 0.05, at a dose of 0.7 microgram/ml) while activin, which has opposite biological effects to inhibin in many systems did not significantly stimulate oocyte maturation at a dose of 1.12 micrograms/ml. The actions of the three proteins studied were reversible.