Bethamethasone was administered to pregnant rhesus monkeys of 134 to 150 days' gestation. At operative delivery, umbilical venous plasma cortisol concentrations were significantly lower in the treated group than in the control group, indicating that the agent crossed the placenta. In the treated group, accelerated differentiation was present in several fetal organs including lung, liver, kidney, and adrenal gland. Brain histologic changes suggestive of neuronal injury were found in some instances. There were no differences in the weights of these fetal organs except for liver. It was markedly increased in steroid-treated fetuses and this was accompanied by a fourfold rise in total hepatic glycogen content. These observations suggest that in the subhuman fetal primate, the differentiation of fetal organs in addition to lung is enhanced by short-term corticosteroid treatment while growth is not affected.