It is now well recognised that the renin-angiotensin system plays a key role in the control of blood pressure not only through circulating angiotensin II but through its interaction with the autonomic and central nervous systems. Angiotensin-converting enzyme (ACE) inhibitors have proved to be effective in lowering blood pressure in different types of hypertension. This study evaluates the antihypertensive effects of enalapril, a new, potent, long acting ACE inhibitor. 50 patients with uncomplicated essential hypertension were included in 4 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 23 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. The effect on mean blood pressure was similar with enalapril and propranolol (enalapril 117 versus 103 mm Hg and propranolol 115 versus 104 mm Hg); however, the glomerular filtration rate decreased with propranolol (105 versus 87 ml/min; p less than 0.05) and was unaltered with enalapril (102 versus 98 ml/min). Triglycerides rose with propranolol (179 versus 231 mg/dl; p less than 0.05) 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 10mm Hg when added to enalapril. The antihypertensive effect of enalapril was similar to that of propranolol; however, the lowering effect on glomerular filtration rate of propranolol did not occur with enalapril. A slight rise in triglycerides occurred only with propranolol. No significant side effects were observed with either propranolol or enalapril. Used as monotherapy, enalapril normalised blood pressure in 44% of cases. Addition of low doses of hydrochlorothiazide significantly increased the antihypertensive effect of enalapril.