The plasma levels of estradiol and progesterone in 5 mature female Japanese monkeys (Macaca fuscata fuscata) throughout a breeding and nonbreeding season were measured by radioimmunoassay. As the breeding season approached, plasma estradiol levels started to increase and a few elevations were detected. However, neither a plasma progesterone rise nor menstrual bleeding was seen. Menstrual bleeding began in December and recurred regularly 4 or 5 times at 4 week intervals until April. During this period, cyclic estradiol elevations (150-250 pg/ml) and luteal progesterone rises (2.0-5.3 ng/ml) were observed. After the last menstruation in March or April, two animals showed an estradiol elevation, but progesterone levels remained in the basal range and amenorrhea persisted. These data indicated that female Japanese monkeys ovulate 4 or 5 times a year, i.e., their breeding season is definitely defined. The changes of both steroids in transitional periods between nonbreeding and breeding, and breeding and nonbreeding seasons, suggested that some follicle development occurred in these periods. The elucidation of the mechanisms involved in the seasonal variation of reproductive functions in the Japanese monkey may provide useful information concerning some aspects of amenorrhea in human females.