An analysis of the pressure pulses in the aorta and 2 ventricles of the heart-lung was made with optically recording manometers of high figures of merit. It was found that the contour of these pulses resembles those recorded in the intact animals. The ratio, systole duration: cycle duration, does not coincide with the standard curve established for the intact animal but is widely scattered about this line. The amplitude of the aortic and intraventricular pressure curves, the mean blood pressure, the initial tension of the left ventricle, and the duration of ejection and total systole were increased when the venous return was increased; the isometric contraction phase, however, was decreased. An attempt was made to keep the mean blood pressure constant as the venous return was increased, but it was apparent that this did not make the arterial load constant, for the systolic and diastolic pressures were not brought back to their former levels. The effect of changing the arterial resistance was not constant. This inconstancy is attributed to the variability in the balance between the effect of the resistance change and the effect of the alteration in initial tension of the ventricle which accompanies it. When cardiac failure was imminent, an increase in venous return produced an effect opposite to the normal. It caused an abbreviation of systole, which preceded the actual decrease in amplitude of contraction. An asynchronism in the onset and offset of left and right ventricular systole similar to that in the intact animal was observed in the heart-lung preparation. The results with the heart-lung preparation indicate that while the preparation eliminates certain variables, it introduces others and does not allow the independent variation of the 2 important factors, the initial tension of the left ventricle and the arterial load.