Seventeen placentae from second trimester abortions induced by the injection of extra-amniotic prostaglandin have been examined. The extraplacental membranes, the chorionic plate of the placenta, the placental intervillous space and the placental villi were normal in all cases. In all but one specimen lesions were, however, noted in the decidua, these including focal decidual necrosis, intradecidual haemorrhage, thrombosis of maternal vessels and dilatation and congestion of the maternal vasculature. Similar decidual lesions were also seen in placentae from spontaneous and surgical second trimester abortions and it is concluded that prostaglandins do not produce any specific lesions in the placenta or decidua. It is suggested that the negative findings of this study indicate that prostaglandins do not induce abortion by either direct injury to the placenta or by provoking utero-placental ischaemia.