The effects of the beta-adernergic blocking drug acebutolol were studied in 23 patients with angina pectoris and angiographically documented coronary artery disease. Patients were evaluated clinically, by graded treadmill testing and by 24-hour Holter monitoring in the control state, after 2 weeks treatment with placebo, and after 2 weeks treatment with 600 mg. and then 1,200 mg. of acebutolol. Acebutolol (in a daily dose of 600 mg.) was an effective antianginal drug: the number of clinical attacks of angina pectoris (p less than 0.001) and the consumption of sublingual nitrate decreased (p less than 0.01), there was a significant increase in the treadmill effort tolerance as measured by the time to appearance of ischemic ECG changes (p less than 0.001) and the total work performed (p less than 0.001), and there was also a significant decrease in ischemic ST segment depression on 24-hour Holter monitoring. Treatment with 1,200 mg. acebutolol was associated with a further decrease in heart rate and a further improvement in effort tolerance on treadmill testing (p less than 0.05). On the large dose of the drug, however, there was no further clinical improvement, and no further improvement on 24-hour ECG monitoring; several patients complained of weakness and fatigue. Graded treadmill testing was an excellent objective method for assessing physical effort tolerance and its improvement after treatment with the beta-blocking drug. Twenty-four-hour Holter monitoring was a useful and complementary test, especially in patients who stopped exercising on the treadmill because of fatigue or weakness, and especially for assessing the efficacy of beta-blockade in controlling emotionally induced tachycardia and ischemia in the patient's own daily environment.