Twelve patients affected by stable effort angina were included in a double blind, cross-over study of Acebutolol and Placebo; Acebutolol was orally administered in a dose of 200 mg. Each treatment was preceded by a basal exercise test and, after 120 min a second test was performed to evaluate the efficacy of the drug. Acebutolol produced a significant improvement of angina threshold (p less than 0.005); the amount of exercise necessary to induce 0.1 mV ST segment depression was significantly increased by Acebutolol (p less than 0.005). There was a significant reduction in the heart rate (HR) and in double product (DP) both at rest and at the maximal work performed; no significant change in the systolic blood pressure was observed. At the same level of work Acebutolol produced a significant reduction in the ST segment depression (p less than 0.05). However, when ST segment was evaluated at the same level of HR and DP, no difference was found between the drug and the placebo. It was concluded that Acebutolol induces a significant increase in exercise tolerance.