Dilevalol is a new antihypertensive agent that is both a vasodilator, through its beta 2-agonist action, and a nonselective beta antagonist. Two multicenter, double-blind studies were performed: study 1 compared dilevalol administered once-daily with either dilevalol or propranolol every 12 hours; study 2 compared dilevalol administered once daily with placebo. Both studies had a placebo run-in period to establish that the baseline supine diastolic blood pressures were consistent in the mild to moderate severity range (95 to 115 mm Hg) at 2 consecutive visits for study 1 and in the mild severity range (95 to 105 mm Hg) in study 2. Patients then were randomized to the double-blind titration phase, during which doses were titrated over a 9-week period to achieve a supine diastolic blood pressure of less than 90 mm Hg and a decrease from baseline of greater than or equal to 10 mm Hg. Patients were then maintained on a fixed dose for 2 months (study 1) or for 1 month (study 2). Dilevalol given once daily was as effective in reducing supine diastolic blood pressure as dilevalol every 12 hours and propranolol every 12 hours (study 1) and was superior to placebo (p less than 0.001) (study 2). In both studies, dilevalol given once daily was effective and well tolerated. The side-effect profile of dilevalol was similar to that of placebo and different from that of propranolol. Treatment with dilevalol resulted in significantly less fatigue (p less than 0.05), bradycardia (less than 50 beats/minute) and mental depression than with propranolol, but significantly (p less than 0.05) more diarrhea/loose stools.(ABSTRACT TRUNCATED AT 250 WORDS)