The daily administration of 240 to 360 mg of diltiazem lowered blood pressure in a dose-related pattern similar to that seen in patients taking a daily dosage of 50 to 100 mg of atenolol. Sustained-release diltiazem was administered twice daily and atenolol once. Goal blood pressure was defined as less than 90 mm Hg or a reduction of greater than or equal to 10 mm Hg for patients with baseline pressures of 95 to 99 mm Hg in the supine position and was achieved in 60% of diltiazem-treated and 63% of atenolol-treated patients. The mean diltiazem dosage at the end of the study was 329 mg daily; for atenolol it was 80 mg daily. Adverse reactions considered possibly or probably drug related were reported by 26% of diltiazem patients and 38% of atenolol patients. Although both drugs were associated with a slower heart rate, atenolol patients showed a significantly greater negative chronotropic effect. Diltiazem, in a sustained-release form taken twice daily, is as effective as atenolol as a sole antihypertensive agent. It has a favorable side-effect profile and may be a useful alternative antihypertensive medication compared with existing beta-blocker therapy with atenolol.