The hormonal regulation of corpus luteum (CL) function during late pregnancy was studied in hypophysectomized monkeys. Between days 149-154 of gestation, 9 days after hypophysectomy, progesterone in the uteroovarian vein (UOV), uterine vein (UV) and peripheral circulation (P) averaged 179.7 ng/ml, 38.9 ng/ml and 5.5 ng/ml, respectively. Amniotic fluid prolactin ranged from 2150-6700 ng/ml and monkey chorionic somatomammotropin (mCS) in mothers carrying live fetuses ranged from 11.4-30.8 micrograms/ml in the UV and P. Prolactin and monkey chorionic gonadotropin in the UV and P were low or nondetectable as was mCS in 2 mothers carrying dead fetuses. CL function was further studied 7 and 39 days after removal of the fetus alone or both the fetus and placenta. Placental delivery was extremely variable, ranging from 2-greater than 63 days post-fetectomy. Although progesterone was not detectable in the P7 days after cesarean section in those animals in which both fetus and placenta were absent, surprisingly, progesterone was measurable in the UOV (range 1.6-48.2 ng/ml). At 39 days, progesterone was either nondetectable or very low. We have interpreted these data to mean: 1) neither the maternal pituitary gland nor a live fetus is necessary for placental or corpus luteum production of progesterone during late pregnancy, 2) the presence of high levels of circulating prolactin and mCS are apparently not necessary for continued secretion of progesterone from the CL during late pregnancy, 3) the fetoplacental unit may be the source of the luteotropic stimulus of late pregnancy since progesterone in the UOV decreases markedly in the absence of the fetoplacental unit or disruption of the unit brought about by fetectomy, and 4) regression of the CL following cesarean section in hypophysectomized monkeys is exceedingly slow when compared to the precipitous regression characteristic of the CL of the nonfertile menstrual cycle.