Secretion of activin A in Japanese monkeys and chimpanzees during pregnancy and early postpartum periods was investigated, and the results were compared with those in humans. Plasma activin A increased throughout pregnancy in Japanese monkeys, and the level was significantly higher in the third and fourth quarters than in the first quarter. After parturition in the Japanese monkey, circulating activin A decreased to levels seen during the normal menstrual cycle. In chimpanzees, plasma activin A remained low until the third quarter of pregnancy and abruptly increased in the fourth quarter. After parturition in chimpanzees, however, circulating activin A concentrations still remained as high as levels in the fourth quarter until 42 h after parturition, then decreased to nadir levels at 1 mo after parturition, suggesting that activin A in chimpanzees may be secreted from other organs such as uterus in addition to placenta. Positive staining with inhibin/ activin alpha-, betaA-, and betaB-subunit antisera was observed in the cytoplasm of the syncytiotrophoblast of term placenta in the Japanese monkey, chimpanzee, and human. These results demonstrated that circulating activin A gradually increased from early pregnancy in Japanese monkey, whereas an abrupt increase occurred at late pregnancy in chimpanzees. These results also demonstrated that the syncytiotrophoblast of placenta is the source of activin A in Japanese monkeys and chimpanzees as well as humans.