The time between administration of human chorionic gonadotropin (hCG) and follicle aspiration was analyzed for alterations in [3H]uridine incorporation [as an indicator of relative synthesis of ribonucleic acid (RNA)] of oocytes from squirrel monkeys and humans. There was a significant decline in both the uptake and incorporation of [3H]uridine in squirrel monkey oocytes by 36 hr following hCG administration to the animals, as compared with 16 hr after hCG. Similarly, RNA synthesis diminished in oocytes collected 35 hr after hCG in humans, as compared with 12 hr after hCG. This reduction in RNA synthesis of maturing oocytes is similar to that of other mammalian species. This provides evidence for an increased interval between hCG administration and follicle aspiration in order to recover mature oocytes for in vitro fertilization studies.