Peripheral plasma beta-endorphin (BEP) was measured in 10 pregnant monkeys during the third trimester (14-21 weeks gestation, term = 23.5 weeks). The study was intended to identify a late-pregnancy rise in BEP that has been reported in women. Levels rose in late pregnancy only in animals that delivered within a few days of the final sample. When both BEP and beta-lipoprotein (BLP) were measured in a further group of 10 animals, the late-pregnancy BEP rise was not accompanied by a rise in BLP. Multiple regression indicated that plasma BEP was correlated with plasma cortisol but that an independent influence of gestational age could be detected. We conclude that monkeys exhibit a rise in BEP just prior to parturition that is not accompanied by increases in cortisol and BLP that would indicate general maternal pituitary activation.