The basic principle of Synchronized Arterial Counterpulsation (SAC-P), as described by Clauss in 1961, appears to be both simple and sound on first inspection. If the blood is removed from the aorta just before and during ventricular systole, the resistance against which the left ventricle must eject can be reduced. The same volume of blood is then returned during ventricular diastole, possibly raising the aortic diastolic pressure and aiding the peripheral perfusion, particularly the coronary perfusion. Controversy, however, continues regarding the effect of this procedure. In the previous studies on assisted circulation, the authors satisfactorily demonstrated favorable hemodynamical changes during SAC-P, but it was thought that only hemodynamical study could not fully clarify its effect on the myocardium. Therefore, so as to assess the effect of SAC-P, the author has attempted to investigate the changes in the coronary hemodynamics and oxygen metabolism of the myocardium induced by SAC-P and to evaluate its effect in aspect of the myocardial oxygen metabolism. In order to prove the effect of this procedure, such parameters as the coronary sinus blood flow, coronary vascular resistance, exchange of oxygen, lactate and pyruvate between the aortic blood and the myocardium, lactate-pyruvate ratio (L/P ratio), myocardial excess lactate (XL) and redox potential (ΔEh) were determined. Lactate and pyruvate are of interest in the myocardial metabolism for two reasons: (1) The myocardium is the main mammalian tissue known to be capable of utilizing lactate as a substrate for energy production; and (2) lactate and pyruvate hold an important position in relation to anaerobic metabolism. Myocardial excess lactate (XL) and redox potential (ΔEh), calculated by the arterio-venous concentration of the two substrates, have been considered as the indicators of cellular oxygenation and are used quantitatively as a measure of anaerobic rate in myocardial metabolism. Thus, the author has attempted to find out the most useful of all parameters measured, in order to estimate the effect of SAC-P on the myocardium with rapidity and accuracy. The pump used in this study was a Davol Heart Pump Model 8500, which was connected to a cannule inserted into the aortic root through the brachiocephalic artery or the abdominal aorta. After pericardiotomy the catheters for the pressure-measurement and the blood sampling were inserted into the left ventricle and the aorta.