The hemodynamic effects of closed-chest extracorporeal circulation were investigated in normal dogs and in dogs subjected to coronary embolization. In both groups, despite delivery of large volumes of blood into the abdominal aorta through peripheral routes, a rise in proximal aortic pressure could not be achieved; to accomplish this it was necessary to increase vascular resistance by balloon obstruction of the lower abdominal aorta, the distal aorta being supplied with superior vena cava blood. The resulting rise in proximal aortic pressure was accompanied by a diminution in calculated left ventricular work.