Abstract Results are described dealing with comparative values on the metabolism of the naturally perfused heart, the empty artificially perfused beating heart in situ, and the fibrillating and arrested perfused heart. The studies were performed on open and closed chest dogs. Perfusion of the arrested heart in the closed chest animals was accomplished with a new technic employing a specially devised arterial catheter. This permitted perfusion of all coronary arteries and did not necessitate cannulation of the coronary arteries. In the open chest animal, there was a significant decline in the oxygen usage from the rhythmically contracting to the arrested heart. The myocardial oxygen usage of the arrested heart was only slightly less than that of the fibrillating organ. Myocardial extractions of substrates were mostly negative during the control period. This was probably the results of difficulties in experimental technic. In the closed chest animal, no statistical difference existed between the oxygen usage of the empty rhythmically contracting heart and the fibrillating or arrested organ. The oxygen usage in these preparations was between 20 to 30 per cent of that of the naturally perfused heart in situ. Mean myocardial balances of glucose, lactate and ketones were positive during artificial perfusion of the rhythmically contracting heart. These values became negative during cardiac standstill or ventricular fibrillation. The results indicate that the oxygen usage of the heart is primarily determined by its external work; they also suggest that previous values calculated for the mechanical efficiency of the beating heart are too low.