Mean +/- s.d. testosterone concentrations in the peripheral plasma of 21- and 22-day-old male fetuses (1.32 +/- 0.43 ng/ml) were significantly (P less than 0.05) higher than those in the umbilical venous plasma (0.37 +/- 0.08 ng/ml). Testosterone concentrations in umbilical venous plasma of male and female (0.29 +/- 0.06 ng/ml) fetuses and in peripheral plasma of female fetuses (0.36 +/- 0.10 ng/ml) were not significantly different. Androsterone levels measured in umbilical venous plasma of male (11.5 +/- 2.5 ng/ml) and female (12.3 +/- 2.1 ng/ml) fetuses were nearly as high as those in peripheral plasma (males, 12.9 +/- 3.1; females, 13.3 +/- 3.5 ng/ml). There were high concentrations of androsterone in the placentas of male (33 +/- 4 ng/g) and female (33 +/- 5 ng/ml) fetuses, suggesting that this organ is the major source of fetal androsterone. We also conclude that a major part of the testosterone present in female fetuses is secreted by the placentas.