The effects of treadmill exercise (up to 85% of the predicted maximum heart rate) on platelet functions and coagulating activities were studied in 26 normal men. Blood sampling for the measurements were performed from the antecubital vein at rest, at 3, 6, 9, and 12 min during exercise, immediately postexercise, and at 6 and 30 min after exercise. Measurements for blood analysis included the following: platelet sensitivity and percent aggregation to ADP, platelet counts, plasma thromboxane B2 and 6-keto-prostaglandin F1 alpha levels, plasma epinephrine and norepinephrine levels, plasma fibrinogen level, activity of plasma antithrombin III, and of plasma factors VIII, IX, XI, and XII. No significant changes were induced by dynamic leg exercise in platelet sensitivities and the maximum and 3-min percent aggregation. The platelet counts increased during exercise in platelet-rich plasma without a significant change in that in whole blood. During exercise, plasma thromboxane B2 levels showed a tendency to increase, while plasma 6-keto-prostaglandin F1 alpha levels to decrease. Plasma epinephrine levels showed a tendency to increase and norepinephrine levels increased during exercise. Among coagulating factors, factor VIII activities and fibrinogen levels increased without altering activities of factors IX, XI, and XII. Antithrombin III activities also increased during exercise. In spite of significant changes in several coagulating factors, prothrombin time and partial thromboplastin time were not influenced by exercise. In conclusion, dynamic leg exercise of a moderate to high intensity produced a significantly elevated plasma level of factor VIII, fibrinogen, antithrombin III, and catecholamines without affecting the hemostatic balance in normal subjects.