Electrocardiograms recorded with a direct-coupled amplifier and recordings of intramyocardial and intracellular potentials were used to determine the nature of the S-T segment shift after myocardial injury. Immediately after coronary artery ligation in the dog, both true S-T and T-Q changes are produced. Lack of responsiveness to the activation wave in the injured area does not account for the initial change in the S-T segment. Changes in the intracellular action potential can be correlated with the electrocardiographic changes. Immediately after injury, cells in the injured area usually repolarize more rapidly than normal cells and this change is significantly correlated with S-T segment shifts. A decrease in resting potential, generally oc curring later than the action potential changes, is similarly correlated with T-Q segment changes.