The antianginal efficacy of carvedilol, a novel beta-blocking agent with vasodilating action, and atenolol were compared in 12 patients with stable effort angina and a positive stress test response. All patients received single doses of placebo, carvedilol, 25 and 50 mg, and atenolol, 50 mg. Heart rate at rest was reduced by 11 and 12 beats/min with both drugs, but only carvedilol, 50 mg, reduced blood pressure at rest. Both carvedilol, 50 mg, and atenolol, 50 mg, increased mean exercise time (24% and 34%, respectively, compared with placebo), time to angina (35% and 51%, respectively), and time to 1 mm of ST-segment depression (54% and 102%, respectively, p less than 0.05 carvedilol vs atenolol). Carvedilol, 25 mg, produced smaller, directionally similar changes in exercise performance, which did not reach statistical significance except for time to 1 mm of ST depression. Both drugs in the 50-mg dose reduced ST-segment depression similarly at maximal and submaximal work levels and lowered heart rate and rate-pressure product at maximal and submaximal work. Carvedilol, 50 mg alone, significantly lowered maximal systolic pressure and rate-pressure product at 1 mm of ST-segment depression. Despite some evidence of vasodilator activity for carvedilol, there was no significant difference in antianginal efficacy with a conventional beta-blocking drug.