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The oxygen consumption of the beating dog heart, performing no external work, was measured in modified Langendorf preparations. Increasing heart rates, induced by rewarming of the cooled sinus node, were associated with linear increments in oxygen consumption. By extrapolation of these measurements, an average cardiac maintenance metabolism at zero rate of 1.98 cc O2/min. and 100 gm heart weight was found. Above this resting metabolism the energy cost of contraction at zero external work amounted to from .83 to 1.18 cc O2/100 beats and 100 gm heart. Measurements of the cardiac oxygen consumption during constant external work per unit of time and while the same heart was performing no external work directly demonstrated the decreased energy cost of external work at slower heart rates.

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