The menstrual cycle was examined in Japanese monkeys, short-day breeders, with or without nursing babies during the breeding season by determining the change in the plasma concentration of progesterone. Blood samples were obtained weekly from suckling and nonsuckling mothers for 6 months from September to February. Plasma concentrations of progesterone in suckling mothers did not show any apparent fluctuation and remained low throughout the study, while irregular menstruation was observed. In nonsuckling mothers, cyclic changes in plasma progesterone concentration accompanied by regular menstruation occurred from October. Annual birth rates were lower in monkeys housed in individual cages (17.6%) than in free-ranging monkeys in Miyajima colony, near Hiroshima in Japan (37.5%). These results indicate that the suckling stimulus for females with young born in spring is potent enough to suppress ovarian activity during the autumn breeding season.