Five cases of maternal and fetal organs were extracted and assayed for their estrogenic substances and titrated against the placental content. In addition, the organs of 6 fetuses were titrated against their placentas. The livers and adrenals of both mother and fetus were studied because of their known involvement in the cholesterol metabolism. The effect of decomposition of the tissues on the amt. of the estrogens extracted was studied. The source of the estrogens during pregnancy was in the general cholesterol metabolism. Although the placenta played a prominent part, it was not the actual source.