Ultrastructural changes of the placenta in toxemia of pregnancy and in diabetes are described. In toxemia of pregnancy in the placenta definite morphological changes were observed; marked atrophy of the syntitial layer was followed by hyperplasia of the cytotrophoblastie cells. The most expressed changes occurred in the vessel wall: swelling of the endothelium, narrowing of the vessel-lumen, deposition of filamentous material in the cytoplasma. Morphological changes in the placenta in diabetes are moderate as compared to those observed in toxemia. Thickening of the basal membrane seems to be most characteristic. Ultrastructural changes are almost identical to those observed in toxemia. Morphological changes revealed may be responsible for the insuffiziency of the placenta and the intrauterine damage to the foetus.