Concentrations of human chorionic gonadotropin (HCG) and progesterone were measured in the peripheral sera of 101 normal pregnant women between 25 and 41 weeks' gestation. HCG levels rose significantly with advancing gestation in the 43 female-bearers (r equals 0.516, p less than 0.001), whereas the 58 male-bearers showed no change (r equals 0.168, p greater than 0.1). Mean HCG levels were significantly higher in female- than in male-bearers (10.7 plus or minus standard error 1.0 versus 8.0 plus or minus 0.9 International Units per milliliter; p less than 0.05). Progesterone levels rose significantly in both female- and male-bearers. The calculated regression lines and mean levels (female-bearers 9.1 plus or minus 0.5; male-bearers 9.8 plus or minus 0.4 mug per deciliter) were not significantly different. There was no correlation between HCG and progesterone levels in either sex or in the entire group independent of gestational age. It is postulated that the lower HCG levels observed at term in male-bearers may result from an inhibitory influence of the higher progesterone and/or androgen concentrations in the male umbilical arterial circulation.