A large reduction of intracellular potassium activity in depolarized subendocardial Purkinje fibers 24 hours after coronary artery ligation is accompanied by a much smaller increase in intracellular sodium activity. Similar intracellular ionic changes also occur during acute ischemia in ventricular muscle and are consistent with mechanisms based on intracellular acidification, which is known to occur in acutely ischemic muscle. To determine if canine subendocardial Purkinje cells 24 hours after myocardial infarction are also acidic, their intracellular pH, surfacepH, and maximum diastolic potential (MDP) were measured with double-barrelpH-sensitive microelectrodes and compared with control fibers in noninfarcted hearts. In 12 mM bicarbonate Tyrode's solution (5% CO2-95% O2), the average intracellular pH was not significantly different (p>Q.25) for normal tissue (6.83 ± 0.08, SD, MDP= −83.5 ± 3.2 mV), for depolarized Purkinje fibers in in fa ret preparations during the first hour of superfusion (6.88 ± 0.11, MDP=−47.8 ± 11.8 mV), and for partially recovered Purkinje fibers in infarcts averaged over the third to sixth hours of superfusion (6.85 ± 0.12, MDP=−74.S ± 9.6 mV). In 24 mM bicarbonate Tyrode's solution, infarct intracellular pH during both the first hour of superfusion (7.08 ± 0.13, MDP=−57.6 ± 15.7 mV) and during the third to sixth hours of superfusion (7.06 ± 0.15, MDP=−76.5 ± 9.6 raV) was significantly alkaline (p<0.0005) compared with average control pH (6.92 ± 0.12, MDP=82.1 ± 3.7 mV). In 24 mM bicarbonate Tyrode's solution, the intracellular pH did vary with MDP (0.0032 pH units/mV). During superfusion of normal Purkinje fibers with hypoxic Tyrode's solution, intracellular pH acidified by 0.22 pH units as they depolarized. Therefore, intracellular acidification does not seem to be a cause of the depolarization of subendocardial Purkinje cells 24 hours after myocardial infarction.