Glycolysis uncoupled from glucose oxidation is a major reason for the intracellular acidosis that occurs during severe myocardial ischemia. The imbalance between glycolysis and glucose oxidation, and the resultant H⁺ produced from glycolytically derived ATP hydrolysis in the diabetic rat heart is the focus of this study. Isolated working hearts from 6 week streptozotocin diabetic rat hearts were perfused with 11 mM glucose and 1.2 mM palmitate and subjected to a 25 min period of global ischemia. A second series of experiments were also performed in which hearts from control, diabetic, and islet-transplanted diabetic rats were subjected to a 30 min aerobic perfusion, followed by a 60 min period of low-flow ischemia (coronary flow = 0.5 ml/min) and 30 min of aerobic reperfusion. H⁺ production from glucose metabolism was measured throughout the two protocols by simultaneous measurement of glycolysis and glucose oxidation using perfusate labelled with [5- ³H/U-¹⁴C]-glucose. Rates of H⁺ production were calculated by measuring the difference between glycolysis and glucose oxidation. The H⁺ production throughout the perfusion was generally lower in diabetic rat hearts compared to control hearts, while islet-transplantation of diabetic rats increased H⁺ production to rates similar to those seen in control hearts. This occurred primarily due to a dramatic increase in the rates of glycolysis. Despite the difference in H⁺ production between control, diabetic and islet-transplanted diabetic rat hearts, no difference in mRNA levels of the cardiac Na⁺/H⁺-exchanger (NHE-1) was seen. This suggests that alterations in the source of protons (i.e. glucose metabolism) are as important as alterations in the fate of protons, when considering diabetes-induced changes in cellular pH. Furthermore, our data suggests that alterations in Na⁺/H⁺-exchange activity in the diabetic rat heart occur at a post-translational level, possibly due to direct alterations in the sarcolemmal membranes. (Mol Cell Biochem 180: 85–93, 1998)