Enalapril (20 mg daily) was used in the treatment of primary arterial hypertension in 28 patients with the WHO class I and II and diastolic blood pressure ranging from 105 to 120 mm Hg. Hypotensive effect after an initial dose of enalapril occurred in 12 patients (42.8%) in whom a decrease in blood pressure at 1 hr exceeded 10% of the basal value (from means = 169.9/109.1 to 146.7/97.9 mmHg). At 6 weeks positive hypotensive effect was found in 46% of patients. A combination of enalapril with hydrochlorothiazide (25 mg daily) in patients without normalization of the blood pressure level increased the number of the favourably treated to 64%. Of 12 patients with positive effects after the first dose of enalapril only seven patients profited from chronic enalapril monotherapy. In chronic treatment enalapril seemed to be well tolerated and side effects were only sporadic.