Groups of female rhesus monkeys were given a 5-day regimen of im injections of hCG (30, 60, 90, 180, and 360 IU, respectively) beginning 2, 6, 10, or 14 days after the midcycle LH surge (day 0). Serum progesterone concentrations in the day 2 treatment group did not differ markedly from values observed in control monkeys throughout normal menstrual cycles. In contrast, monkeys receiving hCG beginning on days 6, 10 or 14 had immediate significant increases in serum progesterone in response to the first injection of hCG. Dramatic responses were seen in the day 6 and 10 groups (2.5- and 5-fold elevations in serum progesterone, respectively); progesterone values plateaued at about 13 ng/ml during the hCG treatment interval. Monkeys receiving hCG on day 14 had 4-fold elevations in serum progesterone, but concentrations did not exceed 6 ng/ml. Serum estradiol increased significantly after hCG to concentrations between 200-300 pg/ml in all treatment groups; peak values were seen at the time of or in the days immediately after the last hCG injection. Serum testosterone concentrations were not significantly altered by hCG administration at any stage of the luteal phase. hCG did not sustain serum steroid hormone concentrations in monkeys with the corpus luteum removed on day 10 of the luteal phase. A 10-day regimen of increasing hCG doses beginning on day 10 of the luteal phase mimicked the steroid hormone secretion patterns observed in control monkeys during early pregnancy. The data show that the qualitative and quantitative steroidogenic capacities of monkey corpora lutea after a gonadotropin challenge are profoundly affected by luteal age.