The systemic venous system is a dynamic component of the cardiovascular system, and it has a key role in the maintenance of the appropriate filling of the chambers of the heart and hence of the stroke volume. Studies in man and animals have characterized the different mechanisms that control the three major components--the splanchnic, muscle, and cutaneous vascular beds--and have provided information on the changes caused by disease and by chemical agents. Concerning man, much of the information has of necessity been derived from the behavior of the limb veins, the control of which is different from that of the splanchnic capacitance vessels. Studies of the behavior of the splanchnic capacitance vessels are needed before the action of drugs that affect the cardiovascular system of man is fully understood.