The placental transfer of conjugated and nonconjugated estrogens was compared in the pregnant rhesus monkey. Placement of catheters in the maternal and fetal circulation allowed for the sampling of blood after the administration of radiolabeled naturally occurring and synthetic estrogens to mother or fetus. In all cases, nonconjugated-estrogen placental transfer was greater than conjugated-estrogen transfer. Comparison of the conjugated estrogens suggested that diethylstilbestrol-monoglucuronide (DESG) was transferred less efficiently than the estrone-sulfate (E, S). High-performance liquid chromatog-raphy (HPLC) of selected plasma samples revealed that 50-90% of the E, S observed in the maternal circulation was cleaved after fetal administration. In contrast, HPLC of maternal or fetal plasma samples after DESG administration revealed only intact DESG. These results emphasize differences in the placental transfer of the synthetic and naturally occurring estrogen hormones.