The subjects of this multicenter study were 159 patients aged 21 to 76 years with mild to moderate uncomplicated essential hypertension. The patients were randomly assigned to receive up to 40 mg of enalapril daily or 360 mg of sustained-release diltiazem daily for a titration period of eight weeks and then maintenance doses for four weeks. The treatment goal was a supine diastolic blood pressure of less than 90 mmHg or a fall of at least 10 mmHg from baseline. During titration, 62% of the enalapril-treated patients and 51% of the diltiazem-treated patients reached the treatment goal after two weeks, 82% and 74% after four weeks, 87% and 84% after six weeks, and 92% and 87% after eight weeks. During the maintenance period, 85% of the enalapril-treated and 87% of the diltiazem-treated patients maintained the goal blood pressure. Treatment side effects were reported by 21% of the enalapril-treated patients and 29% of the diltiazem-treated patients; treatment was discontinued in two patients from each group because of side effects. It is concluded that both drugs were generally well tolerated and effective in the treatment of adult mild to moderate essential hypertension.