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1 Department of Physiology, Harvard University Medical School, Boston, Massachusetts
Aerobic rates of oxygen consumption and anaerobic rates of lactate production, each as a function of work rate at 23–25 C, were compared in 75 isolated perfused hearts from turtles (Pseudemys scripta). Pressure-volume work at constant heart rate was varied over the range of 2–14 x 105 ergs/min. Control experiments established that turtle hearts are capable of sustained (greater than 15 hr) anaerobic work provided some plasma is present in the perfusion medium. Mechanical performance was unaffected by anoxia (pO2 < 2 mm Hg) except for some decrease in heart rate. Adenosine triphosphate (ATP) demand was the same at any given work rate whether the energy was supplied by oxidative phosphorylation or exclusively by anaerobic glycolysis. The efficiency of the heart, expressed as mechanical work divided by free energy consumed, was therefore independent of the metabolic pathways leading to production of ATP. If the 
F' of ATP hydrolysis is taken to be 7.0 kcal/mole, the efficiency of the working turtle heart is 50 ± 3.4%.
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