Pregnant rhesus monkeys (Macaca mulatta) were hypophysectomized at 8-10 weeks gestation to determine effects on plasma levels of estrone (E1), estradiol-17beta (E2), and progesterone (p). The first group of monkeys was subsequently fetectomized at 107-114 days. After hypophysectomy there was an initial decrease in maternal peripheral plasma E2 followed by a rise to preoperative levels within 4-5 weeks. Plasma levels of E1 and P were not markedly altered. After fetectomy, peripheral estrogen concentrations, especially E2, declined markedly. In the second experimental series, we have examined the effects of maternal hypophysectomy on levels of E1, E2 and P either (1) in both mother and newborn baby or (2) in mother, term fetus and umbilical vein. Groups of hypophysectomized and intact pregnant monkeys (3 each) were delivered by cesarean section at the expected time of parturition. Other hypophysectomized and intact monkeys (2 each) delivered normally. E2 levels were elevated significantly in plasma of hypophysectomized monkeys at the time of cesarean delivery and in newborn babies of hypophysectomized mothers shortly after parturition. Escept for these differences, the maternal hypophysis apparently is not a major factor in the control of E1, E2 and P concentrations in pregnant rhesus monkeys.