Platelet aggregation was studied in aortic and coronary sinus blood samples obtained from 18 patients with coronary artery disease (CAD). Using epinephrine and ADP as aggregating agents, platelet aggregation was lower in coronary venous blood than in aortic blood. In nine patients on long-term propranolol therapy, platelet aggregation was lower in both aortic and coronary venous blood compared to the nine patients not taking propranolol. Four other subjects without angiographic evidence of coronary disease exhibited no difference in platelet aggregation in aortic and coronary sinus blood. These data suggest that platelet aggregation is lower in the coronary venous blood of certain patients with coronary disease and chronic propranolol treatment may reduce aggregation in both aortic and coronary sinus blood.