T4, T3, and reverse T3 (rT3) levels and the free thyroxine index were measured in blood collected from the intervillous space (IVS) after placental expulsion and compared to the values in maternal peripheral blood and in umbilical artery and umbilical vein of 21 clinically normal parturients and their conceptuses. T4 levels in maternal peripheral blood did not differ significantly from T4 levels in the IVS, but were significantly higher than those detected in umbilical vein and artery (p < 0.05). There was no difference in T4 levels between umbilical vein and artery. The free thyroxine index was similar for the maternal compartments (maternal peripheral blood and IVS), but differed significantly from the fetal compartments (umbilical vein and umbilical artery). T3 levels in maternal peripheral blood were significantly higher than in the IVS, both of these values being significantly higher than in the fetal compartments. There was no difference in T3 levels between umbilical vein and artery. rT3 levels of maternal peripheral blood were one third that of the IVS (p < 0.05). rT3 levels of the umbilical vein were 1.5 times higher than those of the IVS (p < 0.05) and 5.2 times higher than those of maternal peripheral blood (p < 0.005). No significant difference was obtained between umbilical vein and artery. The increase in rT3 and the decrease in T3 in the IVS in relation to maternal peripheral blood support the hypothesis that the placenta may preferentially convert T4 to rT3 at the expense of T3. The present data, however, do not permit the identification of the site where this conversion takes place.