We investigated the basis for the alterations in the intracellular potassium and sodium activity occurring in subendocardial Purkinje fibers surviving in 24-hour infarcts by examining ion activities in these Purkinje fibers removed from infarcting hearts at earlier times. Specifically, we examined intracellular potassium activity, sodium activity, and pH at 1 and 3 hours after ligation of the left anterior descending coronary artery, and we correlated the changes in ion activity with changes in maximum diastolic potential. We tested various mechanistic hypotheses relating to how the ion activity changes develop and how they affect membrane potential. We found that intracellular sodium activity in tissue removed 1 hour after ligation was on average already maximally elevated by a factor of 2 over control (19.2±2.0 mM [mean±SEM] versus 9.4±0.4 mM). Potassium activity diminished progressively over the first 24 hours (from normal of 112.0±2.7 to 61.6±2.8 mM), although half of the decrease occurred during the first hour (to 86.8±4.1 mM). Intracellular pH did not change at either 1 or 3 hours. Whereas maximum diastolic potential depolarization exceeded the calculated depolarization of the potassium equilibrium potential by a factor of 2 in 24-hour infarcts, the depolarization at 1 and 3 hours could be more nearly attributed to the loss of potassium. The change in the dependence of maximum diastolic potential on potassium equilibrium potential may be due to changes in membrane conductance caused by ionic or biochemical factors. The changes in ion activity continuously develop during the first day after ligation and may be due to multiple factors and mechanisms. (Circulation Research1992;70:566–575)