The effect of alprenolol on central hemodynamics and exercise tolerance was studied in 14 patients with angina pectoris. Alprenolol decreased the heart rate and cardiac output during exercise without affecting stroke volume. The response to alprenolol was greatest in patients with normal and minimal in patients with a markedly ‘hypokinetic’ circulation. The ventricular filling pressure showed a marked increase in a few patients and a decrease in others. Alprenolol increased exercise tolerance both with respect to the onset of angina pectoris and the appearance of ST alterations on the electrocardiogram. Its primary effect seemed to be reduction in oxygen demand of the myocardium as indicated by the decrease of both heart rate and aortic systolic pressure at a given work intensity. A double-blind trial with oral treatment for a period of three weeks suggested a beneficial effect on the exercise tolerance as tested on a bicycle ergometer. Evaluation of the treatment from the patients’ symptoms alone appeared unreliable.