The antihypertensive effects of the oral converting enzyme inhibitor captopril and of propranolol were evaluated in a single-blind trial of 12 weeks in 19 ambulatory men with moderated essential hypertension (supine diastolic blood pressure [DPB], 100 to 120 mm Hg after receiving placebo for two weeks) whose sodium intake was unrestricted. The captopril group included 12 patients and the propranolol group seven. After the initial dose-finding period of four weeks, supine DBP was significantly reduced in eight patients receiving captopril and in four of the patients receiving propranolol. In these patients DBP decreased throughout the following eight weeks. In the remaining patients from each group, DBP was not reduced by either drug given alone at maximum allowable dosages during dose-finding periods, nor by combined administration in following weeks. No adverse side effects attributable to captopril were noted, except in one patient in whom proteinuria developed after seven weeks. Captopril has potential value in the treatment of moderate essential hypertension.