The concept of introducing energy into portions of the vascular system in such a way as to reduce the work of the heart and increase cardiac output is based on sound physiologic and physical principles. The theory has several facets: That the accumulator (arterial) portion of the vascular bed which normally serves to store the great part of the ventricular energy output passively can be activated by the introduction of external energy during diastole by synchronous pulsation of the extramural pressure. Energy storage during systole is accomplished by the reduction of the extramural pressure rather than the increase of intra-vascular pressure which requires ventricular work. That effective driving pressure differentials can be created by regional modification of extramural pressure synchronous with cardiac action. That cardiac contractility requirements can be reduced directly by the synchronous modification of the intrathoracic pressure which is the extramural pressure for the right and left ventricle. That the capacity of the venous bed and venous return can be controlled by varying the mean or minimum value of a synchronously pulsating extramural pressure. Several modalities of this basic concept are under investigation and have been reported previously. Right ventricular assist by synchronous respiration was presented in a preliminary report by our group(l). Osborne(2) and Dennis(3) have both reported their work on left ventricular assist by external pressure on the lower portion of the body. This paper presents our studies of left ventricular assist by synchronous pulsation of the pressure external to the hind quarters of normal dogs. Our system makes use of a rigid chamber and hydraulic coupling which has enabled us to study not only positive pressure pulses but also the effects of negative ambient pressures on venous return, venous pumping, cardiac output and aortic pressure modification.