PIP: To study the plasma patterns of estrogen and progesterone under gonadotropic stimulation simulating early pregnancy, 4 normal female rhesus monkeys (Macaca mulatta) were injected intramuscularly with 50-250 IU of human chorionic gonadotropin (HCG) for 5-10 days during the luteal phase of their menstrual cycle. Plasma levels of estrogen and progesterone were measured by radioimmunoassay of blood samples taken before and after the HCG injections. The exogenous HCG was found to prolong the life span of the corpus luteum and extended the length of the luteal phase in direct relation to the length of HCG treatment. Within 6 hours after the first HCG injection, a 10-fold increase in plasma progesterone was found in all 4 monkeys, and plasma progesterone levels remained comparatively high for 4-5 days after the first injection. The plasma levels of estrone and estradiol were significantly higher than pretreatment levels and normal luteal phase levels during and after HCG injection, with estrone predominating over estradiol levels. It is concluded that HCG injections in the luteal phase stimulate estrogen and progesterone production, with progesterone increasing before estrogen, but with the increased estrogen levels being maintained for longer periods of time.