Physical conditioning appears to protect against the development of vascular disease. Although physical training often evokes favorable alterations in established cardiovascular risk factors, such as plasma lipids and lipoproteins, the metabolic sequelae of regular exercise that mediate a reduction in the risk of cardiovascular disease remain incompletely understood. Studies in recent years have shown physical training to have beneficial effects on blood coagulation and fibrinolytic activity. On general the data support the concept that blood clotting is potentiated by exercise. Mechanisms involved are an increased release of thromboplastine of tissue, increased coagulation with lactate accumulation during exercise, increased concentrations of plasma proteins owing to hemoconcentration, increased concentrations of specific clotting factors, e.g., Factor VIII and fibrinogen, and an alteration in platelet count and platelet function. The acceleration in coagulation is less in the well-exercised individual. There is evidence that an epinephrine mediated mechanism is responsible for the difference between individuals who have a lot of exercise and those who do not. Fibrinolytic activity seems to increase with exercise in a linear relationship with the heart rate during physical activity. An enhancement of the plasma fibrinolytic activity, stimulated experimentally by thrombotic stress such as venous occlusion, could be an important mechanism in the beneficial effect of habitual physical exercise on the risk of cardiovascular disease.