The therapeutic profile of enalapril in mild to moderate uncomplicated essential hypertension was assessed in 265 patients who participated in a multicenter, open-label, prospective study lasting eight weeks. There were 54 younger (aged 39 years or less), 136 middle-aged (40 to 59 years), and 75 older patients (60 years or over). Monotherapy with enalapril in a single daily dosage regimen ranging between 5 and 40 mg resulted in normotension (in the sitting position) in 73% of the younger, 50% of the middle-aged, and 56% of the older patients. Normotension was achieved with 5 mg/day of enalapril in 41%, 18%, and 37% of the subgroups, respectively. Both systolic and diastolic pressures at the end of eight weeks of treatment were significantly lower (P less than 0.01) in the younger patients than in the other two age groups. White patients had significantly greater (P less than 0.001) response of both systolic and diastolic blood pressures than did black patients and required significantly smaller (P less than 0.01) average daily dosages of enalapril (14 mg versus 22 mg, respectively). The overall incidence of side effects was 14% among all 276 patients enrolled in the study. Most were mild and transient, but six patients discontinued enalapril during the first week of therapy because of side effects. There were no cases of rash, dysgeusia, hematological disorders, or deterioration in renal function, but there were two cases of pruritus, one of glossitis associated with an upper respiratory infection, and three of dry cough or wheezing. Angioedema was not observed. Monotherapy with enalapril, usually in a single daily dose of 10 to 20 mg, was effective in inducing normotension in approximately half of the middle-aged and older hypertensive individuals and in nearly three fourths of those below age 40. In this study it was generally well tolerated, with a relatively small incidence of side effects.