Seven adult female rhesus monkeys were laparotomized at days 22, 42, and 157 of pregnancy and blood was collected from the uterine vein and peripheral circulation. Plasma samples were analyzed for monkey chorionic gonadotropin (mCG), monkey chorionic somatomammotropin (mCS), and prolactin by radioimmunoassay. Levels of mCG at day 22 of pregnancy were approximately 250 ng/ml; however, during the later stages of gestation mCG was either nondetectable or less than 0.7 ng/ml. There was no statistical difference in prolactin concentrations between days 22 and 42 of pregnancy, mean levels being between 176-424 ng/ml, but by day 157 prolactin levels of greater than 2,000 ng/ml were recorded. No statistical difference existed between peripheral and uterine vein concentrations of either mCG or prolactin at any of the stages of gestation examined. At day 22, mCS was not detectable; however, at day 42 of gestation mCS titers averaged 1.5 micrograms/ml and 2.3 micrograms/ml in the peripheral and uterine vein plasma, respectively. A statistically significant mCS increase occurred by day 157, levels in the periphery and uterine vein averaging 11.0 micrograms/ml and 16.3 micrograms/ml, respectively. Uterine vein titers of mCS were significantly higher than peripheral titers at both days 42 and 157. Thus, the highest levels of mCG were present during early pregnancy, whereas the highest levels of mCS and prolactin were present during late pregnancy.