Little is known concerning the proliferation of the ovarian surface epithelium or the factors which control this process. To define when and under what circumstances this epithelium proliferates, we have studied the proliferation of mouse ovarian surface epithelium (OSE) during embryogenesis, early postnatal life, various physiological circumstances in the adult and in response to gonadotropic hormones, using the bromodeoxyuridine technique. Proliferation of the OSE is greatest during embryonic development, and falls gradually after birth until sexual maturity is reached. Very little proliferation of the OSE is detectable in adult life in non-pregnant, pregnant or lactating mice. The basal proliferation of the OSE can be increased significantly by inducing follicular development with pregnant mare serum gonadotropin (PMSG) or by administration of the pure recombinant gonadotropins follicle-stimulating hormone (FSH) or luteinizing hormone (LH). These results show that administration of gonadotropins to sexually mature mice induces proliferation of ovarian surface epithelium concurrently with the process of folliculogenesis.