We investigated the characteristics of the monodeiodination of thyroxine to T3 and rT3 in human placentas which were obtained at normal delivery. The placentas were homogenized in a cold sucrose Tris-HCl buffer, pH 7.5. The microsomal fraction was incubated at 37 degrees C in air for 1 hr with 2 micrograms of T4 in the presence of 50mM DTT. The T3 and rT3 generated in the reaction mixture were extracted into cold ethanol and measured by RIA. Among the usal subcellular fractions of the placental homogenate, microsomes were the most potent in deiodinating T4 to rT3. In microsomes, production of rT3 increased with protein concentration, incubation temperature up to 37 degrees C, incubation time up to 120 min and T4 concentration up to 16 micrograms/tube. The production of rT3 from T4 was lost by prior heating of the microsomal fraction to 56 degrees C for 30 min. The net production rate of T4 to rT3 in the microsomal fraction was 17.9 ng/mg protein/micrograms T4/60 min at pH 7.5. RT3 production from T4 was maximal at pH 7.0. The production of T3 from T4 was negligible in the present system. Degradation of T3 in the placentas was rapid. Although the addition of anti-T3 antibody to the reaction mixture suppressed the degradation of T3, it had no effect on the net production of T3, suggesting that the obtained net T3 production rate had not been influenced by its degradation. Degradation of rT3 was negligible. These results indicate that the human placenta actively deiodinates T4 to rT3 enzymatically. This enzyme system might have some influence on the transplacental passage of the thyroid hormone from the mother to the fetus.