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1. The results of the administration of the nonapeptide inhibitor (SQ 20,881) of the enzymatic conversion of angiotensin I into angiotensin II in twelve severe hypertensive patients are presented. 2. Administration of the compound was associated with a fall in blood pressure in ten of twelve patients. 3. Four patients had a normal plasma renin activity (PRA) with a range of 1.6–3.7 ng h-1 ml-1 and eight patients had a high PRA with a range of 5.0–74 ng h-1 ml-1. Two of the patients with normal PRA had no fall in blood pressure despite receiving 2 mg/kg of the compound. Two patients with normal PRA, however, did respond, thus indicating that a high PRA is not necessary for a response to the inhibitor compound. 4. It was found that haemodialysis or diuresis with frusemide enhanced the blood pressure response to the compound. 5. The presence of a measured low total blood volume was found to be associated with an exaggerated fall in blood pressure to a small dose of compound (0.125 mg/kg) in one patient.

1. The anti-hypertensive effect of converting enzyme inhibition was evaluated in twenty-three hypertensive patients (seven renovascular, four essential, four malignant, one scleroderma, three chronic renal failure, four primary or idiopathic aldosteronism). 2. In sixteen patients a single injection (1–4 mg/kg) of the inhibitor produced an immediate anti-hypertensive effect, which lasted up to 16 h. In six patients the anti-hypertensive effect of the inhibitor was significantly improved after sodium depletion. 3. Plasma renin activities increased and plasma aldosterone concentrations decreased consistently except in idiopathic aldosteronism. 4. Converting enzyme inhibition provides a direct way of defining the degree of renin-dependency of the hypertension. Accordingly, it can be used diagnostically and for planning appropriate therapy. Therapeutically, it could be advantageous in hypertensive emergencies because of its safety, specificity and capacity to reduce aldosterone secretion.