1 Fifty-seven patients with mild or moderate essential hypertension, mean age 50 (range 31-69) were randomised to treatment with either captopril or atenolol. Twenty-six patients in each group completed the study. 2 Captopril (25-50 mg three times daily) and atenolol (50-100 mg once daily) caused a highly significant fall in blood pressure both supine and standing. 3 When hydrochlorothiazide (25-50 mg once daily) was added a further fall in blood pressure was observed in both groups. 4 Captopril as single drug caused no significant change in heart rate, while atenolol significantly reduced heart rate both supine and standing. 5 Two patients were excluded from the captopril group, one because of a reversible loss of taste and the other because of dizziness. Three patients were excluded from the atenolol group, two because of bradyarrhythmias and one because of inadequate blood pressure response. 6 Both captopril and atenolol were found to be effective antihypertensive agents, suitable for the treatment of essential hypertension.