Sar¹-ala⁸-angiotensin II, the octapeptide competitive inhibitor of angiotensin II, was administered intravenously to twelve patients with either renovascular, advanced, or malignant hypertension, or with hypertension of pyelonephritis. In eight patients with increased plasma-renin, angiotensin blockade induced immediate striking and sustained reductions in blood-pressure to nearly normal levels. Large excesses of the drug never produced hypotension. This evidence suggests that angiotensin II was involved in sustaining the high blood-pressure. In contrast, in four patients with either low or normal plasma-renin, the angiotensin inhibitor did not alter blood-pressure. Efficacy of the drug was indicated by the striking increases in plasma-renin induced in all patients. Angiotensin blockade with its apparent lack of toxicity provides a new, safe, more specific and effective approach to diagnosis and treatment of hypertensive situations associated with high renin levels. Also, it seems useful for predicting surgical curability of renovascular hypertension.