Twenty patients with severe arterial hypertension were treated with Captopril (an inhibitor of angiotensin 1 converting enzyme). Decrease in blood pressure occurred early and was sometimes important. Significant changes in plasma renin activity, plasma aldosterone and converting enzyme activity were observed. There was a significant inverse correlation between the action of Captopril on blood pressure and the level of extra-cellular volumes. Eighteen patients received long-term treatment (m = 10.05 +/- 1.32 mo.). Side-effects consisted of positive antinuclear antibodies and a possible kidney failure in cases with stenosis of the renal artery.