Whole-day automated ambulatory blood pressure (BP) monitoring was used to assess the duration of the antihypertensive actions of the beta-blockers atenolol (50 to 100 mg; n = 20) and acebutolol (400 to 800 mg; n = 19) each given once daily at 9 AM. When compared with its pretreatment 24-hour average, atenolol decreased diastolic BP by 10 +/- 2 mm Hg (p less than 0.01) and systolic BP by 12 +/- 2 mm Hg (p less than 0.01). Acebutolol decreased the 24-hour diastolic BP by 11 +/- 1 mm Hg (p less than 0.01) and systolic BP by 13 +/- 2 mm Hg (p less than 0.01). More specifically, a comparison of the two drugs during the final 6 hours (3 AM to 9 AM) of the dosing interval showed that the mean decrease in diastolic BP of 10.2 +/- 1.5 mm Hg with acebutolol was greater (p less than 0.05) than the decrease of 6.2 +/- 1.3 mm Hg with atenolol. Moreover, this final 6-hour effect of atenolol was less (p less than 0.01) than that observed during the first 18 hours of the day. The late effects of acebutolol did not change significantly from its early effects. The two agents also differed in their trough (final 2-hour decrease in diastolic BP) and peak (maximum 2-hour decrease in diastolic BP) effects: for atenolol the peak-to-trough difference was 7.8 +/- 3.1 mm Hg (p less than 0.05), whereas for acebutolol it was 3.8 +/- 4.2 mm Hg (N.S.). This study confirms the efficacy of atenolol and acebutolol.(ABSTRACT TRUNCATED AT 250 WORDS)