At the electronoptical level the placental villi of the pre-eclamptic woman are characterized by focal syncytial necrosis, loss and distortion of microvilli, dilatation of syncytial rough endoplasmic reticulum, decreased syncytial pinocytotic activity, a reduced number of syncytial secretory droplets, cytotrophoblastic hyperplasia, degeneration of occasional cytotrophoblastic cells, thickening of the trophoblastic basement membrane and the presence of small fetal capillaries with bulbous endothelial cells. Ultrahistochemical studies show reduced alkaline phosphatase and dehydrogenase activity in the syncytiotrophoblast but an increased acid phosphatase activity. It is suggested that all the observed morphological abnormalities, with the exception of cytotrophoblast cell degeneration, are explicable solely on the basis of utero-placental ischaemia and that no other aetiological factor need be invoked: the cause of the cytotrophoblast cell degeneration is, however, unknown. The ultrastructural findings indicate that there is a decrease in the transfer and synthetic activity of the trophoblast and that cellular respiration is also probably depressed. Nevertheless, there is some evidence that changes of a compensatory nature, designed to limit the effects of the tissue damage, are brought into play.