The circulation of blood during upright exercise involves a two-pump system, a central cardiac pump and a peripheral pump responsible for systemic venous return. Axiomatically, the function of the two pumps must be equal, and certain evidence suggests that the peripheral pump may "drive" the circulation during exercise. Despite its potential importance to circulatory reserve and aerobic fitness, little is known regarding the determinants of the peripheral pump. Pumping function of skeletal muscle and the suction effect of the left ventricle presumably are principal factors in defining systemic venous return. This review, which focuses on data in humans, examines current information regarding the peripheral pump and its potential role as a critical determinant of maximal cardiac output, maximal oxygen uptake, and endurance fitness.