The aim of the present study was to investigate the effects of acute exercise on platelet aggregability, blood coagulation, and fibrinolysis in patients with recent myocardial infarction, and to examine these effects in relation to two different antithrombotic regimens. Forty patients (mean age 60 years) were investigated 3 months after a myocardial infarction. They were randomized to antithrombotic treatment with either warfarin (INR 2.8-4.2) or aspirin 160 mg daily. They performed a standardized ergometer bicycle exercise test. Blood was drawn before and after the exercise. The platelet function tests included a platelet aggregate ratio (PAR), which, in the presence of aggregates, is<1. The coagulation products remained largely unchanged during the exercise, whereas the fibrinolytic activity and the catecholamine levels increased significantly. At baseline, PAR was lower in the warfarin group than in the aspirin group. During exercise, PAR was significantly reduced in both study groups (0.75 vs. 0.80), indicating increased platelet aggregability. Beta-thromboglobulin decreased in both groups. The increased platelet aggregability after exercise despite aspirin is probably due to activation by catecholamines. This implies that aspirin may have a limited antithrombotic effect during physical exercise and probably also in other situations with increased catecholamine levels.