Oxygen consumption and coronary blood flow were measured in anesthetized dogs while cardiac work was altered by changing arterial pressure or by opening an aorta to left atrial shunt. When oxygen consumption during pressure work was compared with that during flow work, at a constant heart rate, it was found that flow work increased oxygen consumption as much, or more, than did increasing pressure work. Coronary blood flow, but not A-V oxygen difference, was correlated with oxygen consumption. The highest correlation with oxygen consumption, however, was obtained for left ventricular end-diastolic pressure, which was even more highly correlated than was cardiac work. The conclusion is that it may be the initial stretch of the myocardial fiber, rather than the arterial pressure or the cardiac output, that is the primary determinant of myocardial oxygen consumption at a constant heart rate.