In a double-blind, crossover study in 20 patients with stable angina pectoris, the effects of long-acting propranolol, 160 mg administered once daily for 4 weeks, were compared with those of standard propranolol, 40 mg given four times daily for 4 weeks. The patients suffered no adverse effects when they were switched between treatment schedules. The average number of episodes of angina during the 4 weeks on long-acting propranolol was 7.3 and on standard propranolol. 6.3. Average nitroglycerin consumption was 5.8 and 4.9 tablets during therapy with these two drug programs. The resting values for heart rate, systolic blood pressure and rate-pressure product were similar when determined 25.4 hours after a dose of long-acting propranolol and 10.7 hours after standard propranolol. When the patients exercised at these times, patients on long-acting propranolol and standard propranolol had similar walking times to the onset of angina and to the development of moderate angina. The values for heart rate, systolic blood pressure and rate-pressure product were similar at rest and during exercise during these two treatment programs. We conclude that long-acting propranolol administered in a dose of 160 mg daily is as effective as 50 mg of standard propranolol four times daily.