Changes in venous tone produced by partial occlusion of the thoracic inferior vena cava, or of the mitral orifice, have been studied in acute experiments on anesthetized dogs, employing distensibility diagrams obtained from injections into the veins of a loop of intestine to measure venous tone. The results demonstrate a reflex mechanism producing venous dilation in response to venous congestion which is mediated by vagal afferents and inhibition of sympathetic efferents.