The basis for the reduced maximum diastolic potential of canine cardiac subendocardial Purkinje fibers surviving one day after extensive transmural infarction was investigated, using double-barrel potassium and sodium ion-sensitive microelectrodes. The maximum diastolic potential of Purkinje fibers in infarct preparations from the left ventricular apex measured during the first hour of superfusion in a tissue bath was −50.1 ± 13.7 mV, a value markedly reduced from the value in control Purkinje fibers from noninfarcted preparations (−85.0 ± 4.5 mV). The intracellular potassium ion activity was reduced by 50.4 mM during this time (intracellular potassium ion activity equals 61.6 ± 16.1 mM, as compared to control intracellular potassium ion activity of 112 ± 19.8 mM). The potassium equilibrium potential was reduced by 16.0 mV (from −97.2 ±4.7 mV in controls to −81.2 ± 6.9 mV), thus accounting for about one half of the reduction in the maximum diastolic potential. After 6 hours of superfusion, the maximum diastolic potential increased to − 78.9 ± 8.7 mV (still significantly less than control). The potassium equilibrium potential had largely recovered (− 93.8 ± 5.9 mV). The intracellular sodium ion activity of Purkinje fibers in the infarcts (15.6 ± 6.9 mM) was elevated during the first hour of superfusion by 6.2 mM compared to control (9.4 ± 2.6 mM), and this was only 12% as much as the initial intracellular potassium ion activity decrease. Sodium ion activity after 3–6 hours of superfusion was not significantly different than normal (12.1 ± 4.9 mM). In conclusion, only a portion of the maximum diastolic potential changes can be explained by a reduction of the potassium equilibrium potential. It is likely that change(s) in the cell membrane sodium-potassium pump's function and in the membrane conductance are also involved. Furthermore, the lack of a compensatory increase in intracellular sodium ion activity accompanying the large reduction of intracellular potassium ion activity may be a consequence of the cellular acidosis, which is known to occur during myocardial ischemia.