The aim of this study was to evaluate utilization of chemical energy in relation to myocardial mechanics in variably afterloaded contractions of cardiac muscle by determining total energy utilization (∼P) in the absence of energy production. Right ventricular papillary muscles of cats were equilibrated at 26°C in a myograph in Krebs' solution while contracting isometrically (12/min). Following treatment with iodoacetate and N2to inhibit completely ATP production from glycolytic and aerobic metabolism, the muscles were stimulated to contract isotonically 20 to 75 times with varying after loads. They were then rapidly frozen in liquid N2-cooled isopentane, and concentrations of ATP, creatine phosphate, inorganic phosphate and creatine were measured.The efficiency of energy utilization for the performance of internal work was 0.0067 μmoles ∼P/g-cm of work and for external work was 0.0031 μmoles ∼P/g-cm. In addition, resting energy utilization was 0.662 μmoles/g/min and activation energy was estimated to be 0.040 μmoles/g/contraction. These findings provide a demonstration of the Fenn effect in cardiac muscle and explain the well-known discrepancy in energy cost when cardiac work is increased by increasing pressure load as opposed to increasing volume load.