This study evaluates the antihypertensive effects of enalapril, a new, potent, long acting angiotensin converting enzyme inhibitor. 69 patients with uncomplicated essential hypertension were included in 5 groups. Group I was used to compare the effects of enalapril and propranolol on blood pressure, renal function, plasma renin activity, aldosterone excretion and plasma lipids in 24 patients after 12 weeks. Group II was used to evaluate long term effects (48 weeks) of these drugs in 13 patients. Group III included 32 patients that received enalapril as monotherapy for 6 to 12 weeks. Group IV was studied to estimate the antihypertensive effect of low doses of hydrochlorothiazide in 18 patients receiving enalapril. Group V was used to compare the antihypertensive effect of hydrochlorothiazide, enalapril or enalapril plus hydrochlorothiazide in 19 patients. The effect on mean blood pressure was similar with enalapril and propranolol (117 versus 103 mmHg and 115 versus 104 mmHg respectively); however, glomerular filtration rate decreased with propranolol (105 versus 87 ml/min; p less than .05) and was unaltered with enalapril (102 versus 98 ml/min). Triglycerides rose with propranolol (179 versus 231 mg/dl; p less than .05) and did not change with enalapril (157 versus 121 mg/dl). In the long term, antihypertensive effects were similar and no significant side effects were observed. In 14/32 patients blood pressure became normal with enalapril alone. Low doses of hydrochlorothiazide (12.5 to 25 mg) decreased mean blood pressure by 10 mmHg when added to enalapril. The antihypertensive effect of enalapril plus hydrochlorothiazide was significant greater than that of enalapril or hydrochlorothiazide alone. Used as monotherapy, enalapril normalised blood pressure in 44% of cases. Addition of low doses of hydrochlorothiazide significantly increased the antihypertensive effect of enalapril. These results show that enalapril is a good antihypertensive agent alone or with low doses of diuretic.