This study was designed to explore the relationship of estrogen, human chorionic gonadotropin (HCG), and food availability to endocytosis in developing oocytes. When estrogen alone is administered to an animal, large amounts of vitellogenin are synthesized by the liver and secreted into the circulatory system, where it accumulates. Under these conditions there is no evidence of endocytosis at the surface of the oocytes. Other studies have shown that following HCG injection into estrogen-treated animals, vitellogenin is removed from the circulation and the oocyte surface is highly contoured and displays endocytotic activity. Food deprivation has much the same effect on oocyte endocytosis as does estrogen. When animals are given HCG and subsequently starved for 20 days, developing oocytes show little endocytotic activity. We conclude that HCG acts to promote or stimulate endocytosis in developing oocytes while estrogen and/or starvation inhibits this process.