To evaluate the effectiveness of myocardial protection during cardiac surgery, continuous measurement was made of the intramyocardial carbon dioxide(PCO(2)) level and pH during aortic cross clamping for 60 minutes under extracorporeal circulation in adult mongrel dogs. Also, samples of myocardium were resected at intervals, and the mitochondrial function and ATP level of these samples were measured in order to judge myocardial injury. The intramyocardial PCO(2) level increased linearly immediately after aortic cross clamping, but after an average of 15.1 minutes, the rate of increase declined. At this point, the ATP levels remained at 70% of the level before aortic cross clamping, and no disturbance of mitochondrial function was found. The intramyocardial pH values revealed slight acidosis. Thus, aortic cross clamping for about 15 minutes did not induce ischemia of the myocardium.It was concluded that monitoring intramyocardial PCO(2) and observing its rate of increase could be useful for determining the effectiveness of myocardial protection during a surgical procedure.