We compared the antihypertensive effects of the beta-blocker atenolol and the converting enzyme inhibitor lisinopril during 12 weeks of treatment in patients with mild to moderate essential hypertension. Atenolol (n = 10) significantly decreased conventionally measured blood pressure from 144/103 to 135/93 mm Hg and lisinopril (n = 9) from 150/104 to 130/92 mm Hg. Based on data derived from automated 24-h ambulatory blood pressure monitoring, atenolol decreased the average whole-day systolic pressure by 18 +/- 6 mm Hg (p less than 0.02) and the diastolic pressure by 11 +/- 2 mm Hg (p less than 0.01). Lisinopril produced decreases of 27 +/- 5 mm Hg (p less than 0.01) and 13 +/- 2 mm Hg (p less than 0.001). Examination of the 24-h blood pressure patterns showed that the efficacies of the two drugs were similar. Each appeared to be effective throughout the whole-day monitoring period, although only lisinopril significantly decreased blood pressure during the final four-h period (4 AM to 8 AM) preceding the next day's dose. Neither drug produced significant echocardiographic changes in left ventricular wall thickness or muscle mass during the short-term treatment. Lisinopril and atenolol effectively decrease blood pressure during a 24-h period. Moreover, we found that automated whole-day blood pressure monitoring is a useful tool for comparing the efficacy and duration of action of differing antihypertensive agents.