Microsomes were prepared from placentas of normotensive women and of patients suffering from pregnancy- induced hypertension (PIH). Activity of Na,K-ATPase (estimated as ATP hydrolysis) from the hypertensive tissue was lower than from tissue of normotensive women, even if the number of Na,K-ATPase molecules (monitored by anthroyl ouabain binding) was actually greater in the hypertensive tissue. The affinity of Na,K-ATPase for anthroyl ouabain was about four times higher in plasma membranes of hypertensives, indicating some structural change in the Na,K-ATPase or in its vicinity. Assuming the presence of an endogenous digitalis-like factor, the results suggest a simple way of explaining not only the lower Ma,K-ATPase activity in the placental membranes of hypertensives but also the different extent of enzyme inhibition in different tissues of PIH patients.