We have studied the effects of intravenous infusion of saralasin, a competitive antagonist of angiotensin II, in 27 hypertensive patients: 13 had essential hypertension, 14 had renal lesions which involved the renal artery in 9 cases. In essential hypertensives saralasin administration did not significantly lower blood pressure, even after mild salt depletion. It induced a decrease in blood pressure in 7 patients with renal abnormalities (5 with renal artery stenosis, 2 with unilateral parenchymal disease). It may be suggested that in these cases hypertension was dependent, at least partly, on the renin-angiotensin system. In agreement with other investigators, we have found a relationship between the level of plasma renin activity and the blood pressure decrease obtained by saralasin. In patients with unilateral renal artery stenosis, blood pressure decrease was related to renal vein ratio of plasma renin activity.