We have started an in vitro fertilization program in cynomolgus monkeys in an effort to develop an appropriate animal model to improve our knowledge of early embryonic development. In 16 of 25 animals treated with menopausal gonadotropins, preovulatory follicles developed. Follicular aspiration was performed at laparotomy after human chorionic gonadotropin injection. A total of 299 follicles were aspirated, and 251 oocytes were recovered. Oocytes were cultured in 1 ml of growth medium or 100 microliter droplets of medium under mineral oil. Semen samples were obtained by electroejaculation, and the oocytes were inseminated 4 to 24 hours after aspiration. Culture under mineral oil significantly increased the fertilization and cleavage rates. Of 68 embryos produced, 24 have been used in 10 embryo transfers, resulting in two pregnancies.