Age- and season-related changes in gonadotropin-gonadal functions were studied in captive female Japanese monkeys (Macaca fuscata fuscata), with special reference to reproductive senescence. In experiment 1, a total of 57 nonlactating females at ages 3-32 yr were used. Blood samples were collected for at least one breeding season (September through March), and changes in plasma concentrations of LH, estradiol, and progesterone were examined. In experiment 2, two groups of females (older females, n = 5, over 21 yr old; mature females, n = 5, aged 8-12 yr) received s.c. injections of 50 micrograms/kg of estradiol benzoate during the mid-breeding season. In experiment 1, ovarian functions as well as fecundity rates were maintained in the animals aged 5-20 yr. A decline in ovarian activity became apparent at 21-25 yr of age, when animals exhibited a slight increase in plasma LH during the breeding season. Onset of menopause occurred at around 27 yr of age, near the end of the life span. Menopausal females exhibited a marked seasonal difference in plasma LH concentrations, with high levels during the breeding season. In experiment 2, estradiol treatment elicited an LH surge in both old and mature females. The marked seasonal difference in plasma LH, as well as the maintenance of LH response to estradiol in menopausal females, suggests a possibility that biannual changes in sensitivity of the hypothalamo-hypophysial axis to the negative feedback action of estradiol are maintained even after menopause.