The antihypertensive and biochemical effects of the angiotensin-converting enzyme inhibitor enalapril were compared with those of the thiazide diuretic bendrofluazide. Patients with untreated mild to moderate essential hypertension were entered into a randomized, double-blind, double-dummy, parallel-group study. Blood pressure (BP) decreased significantly (p less than 0.001) with either drug therapy: from 172/103 to 147/87 mm Hg (n = 18) with enalapril and from 179/104 to 165/95 mm Hg (n = 22) with bendrofluazide; thus, enalapril produced a greater reduction (p less than 0.05) than bendrofluazide. The median dose of enalapril was 20 mg/day (range 10 to 40) and that of bendrofluazide was 5 mg/day (range 2.5 to 10). Both drugs reduced serum sodium levels by small amounts. This was significant only for enalapril (decrease of 1.3 mmol/liter, p less than 0.05). Hyponatremia was not seen in any patient. Three patients were withdrawn from each treatment group due to adverse effects, although both drugs were generally well tolerated.