The response of the fibrinolytic and blood-coagulation systems to intense treadmill exercise and the role of the beta-adrenergic receptors in mediating this response were studied in five healthy volunteers. On one day, placebo, and on another, the beta-adrenergic-blocking agent, propranolol, was administered before exercise. Exercise significantly enhanced fibrinolysis, primarily because of increased plasminogen activation. Exercise also caused factor-VIII increases of two to four times the resting values. No significant changes were noted in any other coagulation factors. The administration of propranolol did not affect enhanced fibrinolysis and plasminogen activation but did prevent the increase in factor VIII. Thus, while exercise seems to increase factor-VIII activity by beta-receptor stimulation, this mechanism is not responsible for the associated increase in fibrinolysis.