Ovaries from 4-day-old mice were organ-cultured and the effects of progesterone, androstenedione and estradiol-17β on early follicular development were investigated. Both androstenedione (40-1,000 ng/ml) and estradiol-17β (8-1,000 ng/ml) promoted proliferation of granulosa cells in developing follicles, while progesterone showed no remarkable effect. Cyproterone acetate (4,000 ng/ml), an androgen receptor antagonist, partially inhibited the granulosa cell proliferation induced by androstenedione (40 ng/ml). Dihydrotestosterone (8-1,000 ng/ml), which is not converted to estrogens, also induced proliferation of granulosa cells. These results suggest that androgens directly promote proliferation of granulosa cells in early developing mouse follicles in vitro.