The effect of extracorporeal circulation (ECC) on myocardial myosin adenosine triphosphatase (ATPase) activity was studied in 23 dogs with normal heart and 17 dogs with left ventricular hypertrophy. Intermittent aortic cross clamping for 60 minutes was done in some dogs and not in others during ECC. The cardiac functions and myocardial ATPase activity before and 60 minutes after ECC were compared. Cardiac function was depressed in dogs undergoing aortic cross clamping, especially in those with hypertrophied heart after ECC. It was observed that cardiac myosin ATPase activity was diminished in dogs with hypertrophied left ventricles. In the other dogs, no remarkable changes were observed in the myosin ATPase activity. It was concluded that myosin ATPase activity was depressed during aortic cross clamping in dogs with hypertrophied left ventricles and that the disturbance of energy utilization in the myocardium was one of the causes of depressed cardiac function after ECC.