The response of the ovaries to exogenous gonadotropins was examined by determining serum estradiol (E₂) and progesterone (P) levels and by observing the development of follicles by using 5-to 7-month-old cynomolgus females. The serum E₂ levels began to increase 2 h after a single injection of 15 IU of human menopausal gonadotropin (h MG) and reached peak levels at 6 h after dosing, but the E₂levels decreased to near predosing values at 24 h. The serum E₂ levels increased markedly due to the repeated injections of 15 IU h MG for 10 days (days 0-9), but the P levels were low and equal to the levels observed at the follicular stage in adult females. The E₂ levels decreased after an injection of 1,000 IU h CG on day 10, and the levels on day 21 were almost equal to the predosing values on day 0. On the other hand, the P levels increased transiently on day 16, which suggested ovulation. In the histopathological examination of the ovaries obtained on day 10, large antrum follicles were observed. These results confirmed that the ovary of infant cynomolgus monkeys reacted in the same way as the ovary of adult monkeys to exogenous gonadotropins and suggested that this method could become a useful means of evaluating the ovarian function of infant females.