Therapy with 10 to 40 mg once daily of enalapril, a new angiotensin converting enzyme inhibitor, was compared with therapy with 50 to 100 mg once daily of atenolol in a double-blind randomized multicentre trial in 180 patients with a diastolic blood pressure (determined with the patient seated) of 95 to 115 mm Hg between March 1984 and April 1986. A total of 86 patients (61 men and 25 women with a mean age of 49.4 years and a mean blood pressure [and standard deviation] at entry into the trial of 155.5 [15.7]/101.0 [6.3] mm Hg) received enalapril, and 94 patients (63 men and 31 women with a mean age of 50.9 years and a mean blood pressure at entry of 156.6 [16.6]/101.2 [5.7] mm Hg) received atenolol. After a placebo run-in period the patients received increasing dosages of medication every 2 weeks until the target diastolic blood pressure of 90 mm Hg or less was achieved on two consecutive visits, the maximum dosage was reached, or the patient withdrew because of adverse effects. At 14 weeks the mean blood pressure was 141.6 (18.0)/90.1 (9.5) mm Hg in the enalapril group (61 patients) and 140.0 (17.1)/88.4 (8.7) mm Hg in the atenolol group (54 patients). The target diastolic blood pressure was achieved on completion of therapy (between weeks 10 and 14) in 67 (77%) of the patients receiving enalapril and 75 (79%) of the patients receiving atenolol. Compliance was similar in the two groups. Seven patients withdrew because of adverse effects, three in the enalapril group and four in the atenolol group. The results suggest that once-daily monotherapy with enalapril, 10 to 40 mg, is effective in the treatment of mild to moderate hypertension and is as effective as and tolerated as well as once-daily therapy with atenolol, 50 to 100 mg.