Mechanical devices to assist the failing heart have undergone considerable development during the past several years. Such systems, recently reviewed by Spencer and co-workers(1)', and Schenk and co-workers(2), include those for: left ventricular bypass(3), veno-arterial bypass(4), counter-pulsation(5), which has been accomplished by blood exchange via arterial cannulae, intra-aortic baloons and external body plastic suits, and post-systolic or diastolic augmentation(6). The purpose of this report is to present a diastolic augmentation system which was devised at The Johns Hopkins University School of Medicine(6) and which recently has been technically improved with the cooperation of the Westinghouse Corporation. The following criteria seemed desirable in devising the system to be described. The device must be easily and quickly applied in order to be useful for the urgent situation in which it is needed. A stimulus suitable for triggering an arterial pump in synchrony with the closure of the aortic valve must be easily accessible. For this, a peripheral arterial pulse was found to be convenient and satisfactory as the pump injection could then occur only following the triggering systolic pulse wave, that is, during diastole. An arterial channel suitable for adequate bi-directional blood exchange must be easily accessible. It has been determined that the sudden injection of a small amount of blood into the elastic, semi-closed central arterial tree results in a transient, acute elevation of arterial pressure. The amount of blood passively primed during systole into a blind-end pump connected to the common femoral artery was found to elevate considerably the arterial pressure when returned to the aorta by sudden injection during early diastole.