Occlusion of the thoracic aorta (AO) in dogs with a constant volume right ventricular extracorporeal bypass increased cardiac output (Q) by 43% and mean arterial pressure by 46%, while mean systemic pressure (MSP) was unchanged. We compared AO with occlusion of the brachiocephalic and left subclavian arteries (BSO) which decreased cardiac output by 5%, increased mean arterial pressure by 32%, and increased MSP by 11%. We feel these results confirm that AO elevates preload by transferring blood volume from the splanchnic veins to the vascular system drained by the superior vena cava. If the heart is competent to keep right arterial pressure at or near zero, this increase in preload will elevate Q above control levels. Comparing our data with results of other authors who have not controlled right atrial pressure, emphasizes the importance of a competent right ventricle in allowing venous return to determine Q.