The major vessel occlusion method applied to the supine, anesthetized dog with an open chest has proven useful in elucidating the role of the venous system in the complex phenomenon of venous return. This method momentarily isolates a large part of the venous system from the arterial system and from the action of the heart and the respiratory and muscle pumps. Simultaneous occlusion of the thoracic inferior vena cava and the descending aorta (MVO) divides the dog's circulation into a cephalad zone and a caudad zone. Experimentally induced changes in the status of the cephalad circulation during MVO can affect the caudad zone only through neuronal pathways. Therefore, MVO makes it possible to investigate the responses of the venous system to vascular reflex activity as well as direct and indirect actions of drugs. During the “resting” state, blood returns to the heart because of the existence of a venous pressure gradient of 3 to 4 mm. Hg, operant between the reactive venous reservoir and the central venous conduit (“intra-venous” gradient). Augmentation of sympathetic tone produces an increase in venous return, of venous origin, due to an increase in the effective “intra-venous” pressure gradient from 3 to 4 mm. Hg to 5 to 10 mm. Hg. The ability of the venous system to increase venous return independently, has been shown to be a significant factor in thearterialhypertensive response to carotid occlusion. The venous system can serve as a buffer against the effects of sudden, momentary changes in arterial flow by maintaining, reducing, or augmenting venous return.