The effectiveness of metoprolol in relieving angina pectoris was compared with that of propranolol at equipotent dosages in a series of 20 patients. Before double-blind periods propranolol was given in gradually increased dosages to obtain a maximal effect for each individual patient. The responses were monitored using subjective assessment based on attack rate and nitroglycerin consumption, as well as objective maximal exercise testing. Both compounds significantly increased the exercise tolerance on a bicycle ergometer, and relieved angina as assessed subjectively. No significant differences were found between the responses to metoprolol and propranolol. The mean increase in exercise tolerance amounted to 36% with metoprolol. Biochemical monitoring revealed no harmful effects. Two cases of rebound phenomenon were observed at the transition from metoprolol to placebo. It was concluded that metoprolol is a useful new cardio-selective beta-blocking compound equipotent to propranolol in the treatment of angina pectoris.