In 24 cats with permanent and temporary coronary artery occlusion of 45–120 min duration, the heart was subsequently completely freed from work for 120 min by a total cardiopulmonary bypass with the aid of a heart-lung machine designed for cats. Neither after permanent nor temporary coronary occlusion did the relief of the heart by cardiopulmonary bypass affect the extent of ischemic myocardial damage, recognizable after intravital fluorescent staining with acridine orange, by the estimation of the hydrogen-ion concentration in the tissue and by the macrotest with nitro-BT, without substrate addition. The favorable effect of mechanical circulatory support in acute myocardial infarction, therefore, must be due to relief of the nonischemic myocardium and not to a reduction of the infarcted are