The effects of treadmill exercise on platelet function, blood coagulability and fibrinolytic activity were evaluated in 20 patients with lone atrial fibrillation (AF) and 15 age-matched normal controls (normals). Multistage treadmill exercise up to 85% of the predicted maximal heart rate was performed, and blood for measurements was obtained pre-exercise, and immediately and 6 min post-exercise. There was an increase in the platelet sensitivity to ADP-aggregation after exercise in both groups. Pre-exercise plasma beta-thromboglobulin (beta-TG) levels were higher in AF than in normals. Beta-TG increased after exercise in both groups (immediate post-exercise; 35.1 ng/ml for normals and 62.8 ng/ml for AF), and the increase was greater in AF than in normals. PT and APTT shortened, and plasma fibrinogen levels increased after exercise in both groups. Pre-exercise levels of plasma ATIII and protein C were lower in AF than in normals. These two proteins increased after exercise in both groups. However, the increase was greater in normals. Plasma alpha 2-PI increased after exercise in both groups; the level was lower in AF than in normals at each exercise stage. In conclusion, enhanced platelet activity, and lower levels of anticoagulant and antifibrinolytic activity were observed in AF not only at rest but also after treadmill exercise. These changes might reflect the hypercoagulable state in patients with AF. It is speculated that the risk of thromboembolic complications may be enhanced with exercise in AF patients.