Fifty-six squirrel monkeys were studied to develop means of accurate ovulation induction. Ovaries were exposed to determine ovulation time. Pre-treatment with progesterone to suppress spontaneous ovulation (5 mg daily for 5 days) was followed by : (a) no fsh, (b) 1 mg fsh, (c) human menopausal gonadotrophin (hmg), equivalent to 75 i.u. fsh, or (d) 200 i.u. pregnant mare's serum gonadotrophin (pmsg), each for 4 additional days. Ovulation was induced in 43% of the animals with either 250 or 500 i.u. of human chorionic gonadotrophin (hcg) administered intravenously (i.v.) or intramuscularly (i.m.). Four double ovulations were observed, the others were single ovulations. No follicular development occurred in animals which did not receive an exogenous source of fsh. fsh was superior to hmg and pmsg in its ability to promote follicular growth. Nearly twice as many animals ovulated after 500 i.u. hcg than with 250 i.u. Injection of the hcg i.v. or i.m. did not affect the percentage of animals that ovulated but greater variation in the time of ovulation was noted with i.v. injections.