The translocation of adenine nucleotides across the inner mitochondrial membrane and the tissue concentration of long-chain acyl-CoA esters were studied in dog heart after experimental myocardial ischemia. Ligation of the anterior coronary artery initiated events leading to an early decrease in adenine nucleotide translocase activity. A reciprocal increase in the concentration of heart tissue long-chain acyl-CoA esters was also observed. Adjacent nonischemic tissue showed changes intermediate between that of ischemic and normal heart tissue. It is postulated that a decrease in fatty acid oxidation after myocardial ischemia would lead to an accumulation of long-chain acyl-CoA esters, which in turn would inhibit adenine nucleotide translocation. The net result would be a lowering of the energy charge of the cell, adversely affecting muscle contraction and electrical conduction.