Progesterone levels in the rhesus fetus during the last trimester of pregnancy were higher in the umbilical vein than in the umbilical artery. Moreover, the progesterone concentration in both vessels exceeded that of maternal peripheral plasma. Progesterone levels in the umbilical vein were consistently higher in female fetuses (22.02 ± 3.24) than in male fetuses (7.29 ± 2.14); those in the umbilical artery were also higher in female fetuses (11.05 ± 1.75) than in male fetuses (5.16 ± 1.43). However, progesterone levels in the peripheral plasma of mothers were not affected by the sex of the fetus. The fetal genotype therefore may influence the rate of biosynthesis or the metabolism of progesterone in the rhesus placenta and may affect the fate of progesterone presented to the fetus. (Endocrinology91: 253, 1972)