Ten patients with effort angina were studied in a randomized double-blind cross-over trial in order to investigate the effect of sustained release propranolol (LA) and conventional propranolol (Prop) formulations on exercise tolerance. LA was given once daily and Prop three times daily. Treadmill exercise tests were performed using the Bruce protocol and the plasma propranolol levels were measured. The following results were obtained: LA proved to have an anti-anginal effect similar to Prop. No adverse reactions were observed after administration of either of the drugs. Similar suppressive effect of the heart rate, blood pressure and double product at rest and on exercise was seen after LA and Prop while the exercise capacity was increased. The plasma level of propranolol was higher at 2 hours after Prop than LA administration, but there was no difference between LA and Prop at 4 and 24 hours after administration. The plasma propranolol level at 4 hours after LA correlated with the percent reduction in exercise heart rate and with the percent reduction in the double product. Our study suggested that once-a-day administration of LA could improve patients' compliance and adds another choice to the list of clinically useful beta-blockers for the treatment of angina pectoris on effort.