Puppies' hearts were preserved under hypothermic ischemic conditions. They were initially flushed with an intracellular type solution. It was shown that ATP stores were better maintained if the fluid was acid. The level of ATP at the end of 24 hr is compatible with recovery. In these conditions, the study of lactate production and tissue pH seems to indicate that it is due to a slower ATP decrease rate. The pH of the perfusate might act at the extracellular side of the myocardial cell.