Gonadotropin binding sites were localized by autoradiography after incubation of human ovarian sections with 125I-labeled gonadotropins. The binding sites for 125I-labeled human follicle-stimulating hormone (125I-hFSH) were identified in the granulosa cells and in the newly formed corpora lutea. The 125I-labeled human luteinizing hormone (125I-hLH) binding to the thecal cells increased during follicular maturation, and a dramatic increase was preferentially observed in the granulosa cells of the large preovulatory follicle. In the corpora lutea, the binding of 125I-hLH increased from the early luteal phase and decreased toward the late luteal phase. The changes in 3 beta-hydroxysteroid dehydrogenase activity in the corpora lutea corresponded to the 125I-hLH binding. Thus, the changes in gonadotropin binding sites in the follicles and corpora lutea during the menstrual cycle may help in some important way to regulate human ovarian function.