There was a temporal relationship between ovarian development and the sites to which 125I-labeled gonadotropins became bound. Labeled human LH [luteinizing hormone] and FSH [follicle stimulating hormone] bound uniformly to 5 and 15 day old ovaries. At 21 days, heavy FSH and light LH binding was observed over the granulosa cells of some antral follicles. LH binding to the granulosa cells increased at day 33 and again at day 38. Quantitative determination by .gamma.-ray spectrophotometry of binding to ovarian sections showed that LH binding gradually increased with advancing age, but FSH binding remained relatively constant between days 5 and 33 with a significant increase between days 33 and 38.