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.