Significant increases in myocardial calcium and sodium concentrations and decreases in magnesium and potassium concentrations were observed in infarcted myocardium 5 days after injury when compared against normal myocardium from the same animals. Intermediate changes were observed in periinfarct tissues. Even apparently normal myocardium from infarcted animals showed these same changes when compared against normal myocardium from animals without infarction. <sup>45</sup>Ca uptake into intact myocardium and specifically the mitochondrial fraction were still increased more than threefold during the 24-hour period prior to sacrifice on the fifth postinfarction day. Myocardial calcium appears to be in a readily exchangeable equilibrium with circulating serum calcium even in this late stage in the evolution of the infarction.