The antihypertensive efficacy of sustained-release diltiazem 300 mg at a single daily dose was evaluated and compared to that of enalapril 20 mg at a single daily dose, and to a combination therapy of enalapril 20 mg with sustained-release diltiazem 300 mg in patients with mild to moderate arterial hypertension (supine diastolic blood pressure between 95 and 115 mm Hg). After a washout period and a placebo period, each lasting 2 weeks, 96 patients were randomized in this double-blind study to three parallel groups that were treated for 28 days. In the groups treated with diltiazem, enalapril, or the combination of both, the drop in arterial blood pressure was 20, 11, and 19 mm Hg for supine diastolic blood pressure (supine DBP): 18, 12, and 19 mm Hg for standing diastolic blood pressure (standing DBP); 22, 20, and 23 mm Hg for supine systolic blood pressure (supine SBP); and 21, 19, and 24 mm Hg for standing systolic blood pressure (standing SBP), respectively. The reduction in DBP was significantly more pronounced in the group treated with sustained-release diltiazem 300 mg than in the group treated with enalapril 20 mg. Cardiac tolerance was good for patients treated with sustained-release diltiazem 300 mg alone or the combination therapy: no orthostatic hypotension or lengthening of the PR interval in the electrocardiogram was observed. Although side effects of sustained-release diltiazem 300 mg occurred more frequently, they were not severe and disappeared immediately when treatment was suspended.