Umbilical venous and umbilical arterial progesterone concentrations were determined in 99 pregnancies by radioimmunoassay. The mean +/- SE plasma progesterone in the umbilical vein was 1295 +/- 73 ng/ml, and in the umbilical artery it was 520 +/- 38 ng/ml with an arterio-venous difference of 775 +/- 58 ng/ml and a vein:artery progesterone ratio of 3.6 +/- 0.3. The secretion rate of progesterone into the fetal compartment was calculated to be 84 mg/24 hr. There was no significant difference in these parameters between male and female fetuses, fetal weight, mode of delivery, Apgar score, and duration of labor. It was therefore concluded that the fetal genotype and stress on the fetus did not appear to significantly affect fetal extraction and utilization of progesterone.