The efficacy and safety of enalapril and hydrochlorothiazide was investigated in a multicenter study of 81 patients with mild to moderate essential hypertension. The subjects were randomly assigned to one of three groups and, following a placebo period, given enalapril maleate (20 mg), hydrochlorothiazide (12.5 mg), or a combination of the two drugs (32.5 mg). A significant decrease in blood pressure was observed after only 2 weeks in the enalapril and enalapril-hydrochlorothiazide groups. A double dose was required to achieve a satisfactory response in one-third of the patients in both the enalapril (9 of 27) and the enalapril-hydrochlorothiazide groups (8 of 27). Adverse reactions included cough in one patient and mild hyperkalemia in another, both of whom received enalapril. Two patients on the drug combination developed side effects--symptomatic orthostatic hypotension in one and impotence in the second. Enalapril alone and in combination with hydrochlorothiazide appears to be effective and well tolerated.