Twenty-five patients with essential hypertension were randomly assigned to treatment with either captopril or atenolol. There were 15 males and 10 females and their mean age was 53 years, range 32-66 years. Systolic and diastolic blood pressures were significantly reduced by atenolol 50-100 mg once daily. Captopril 25-50 mg 3 times daily caused a significant decrease in supine diastolic but not in systolic blood pressure. After the addition of hydrochlorothiazide (25-50 mg) to those who did not become normotensive (supine diastolic blood pressure less than 95 mm Hg) on captopril or atenolol alone, blood pressure was further reduced. Captopril combined with hydrochlorothiazide significantly reduced both supine and standing blood pressure by 31/17 mm Hg (p less than 0.01) and 33/18 mm Hg (p less than 0.001) respectively. Atenolol combined with hydrochlorothiazide caused a significant decrease of both recumbent and standing blood pressure by 21/10 mm Hg (p less than 0.01) and 23/13 mm Hg (p less than 0.05 systolic, p less than 0.001 diastolic). The reduction of systolic blood pressure was significantly better with the captopril/hydrochlorothiazide combination, while there was no difference between the groups as regards diastolic blood pressure reduction. Thus, hydrochlorothiazide potentiates the blood pressure lowering effect of captopril more than of atenolol. This could be due to a synergistic interaction between captopril and hydrochlorothiazide.