Captopril was given orally to 24 patients with moderate to severe essential or renovascular hypertension, with variable degrees of sodium-volume depletion. Initiation of treatment resulted in orthostatic hypotension in 5 and in symptoms and signs of hypotension while still recumbent in 4; bradycardia accompanied the adverse hypotension which was relieved by iv atropine. After iv infusion of 1-2 liters of 0.9% NaCl patients cound resume their normal activities and be treated with captopril. The development of hypotension was related to the prevailing plasma renin level, which was partly determined by the degree of sodium-depletion and the aetiology of hypertension. The degree of sodium-depletion, the aetiology and severity of hypertension and heart rate did not contribute independently from plasma renin activity to the development of hypotension.