We have shown previously that preconditioning myocardium with four 5-minute episodes of ischemia and reperfusion dramatically limited the size of infarcts caused by a subsequent 40-minute episode of sustained ischemia. The current study was undertaken to assess whether the same preconditioning protocol slowed the loss of high energy phosphates, limited catabolite accumulation, and/or delayed ultrastructural damage during a sustained ischemic episode. Myocardial metabolites and ultrastructure in the severely ischemic subendocardial regions were compared between control and preconditioned canine hearts. Hearts (four to 10 per group) were excised after 0, 5, 10, 20, or 40 minutes of sustained ischemia. All groups had comparable collateral blood flow. Preconditioned hearts developed ultrastructural injury more slowly than controls; evidence of irreversible injury was observed after 20 minutes in controls but not until 40 minutes in preconditioned hearts. Furthermore, after 40 minutes of ischemia, irreversible injury was homogeneous in controls but only focal in preconditioned myocardium. Preconditioning reduced starting levels of ATP by 29%. Nevertheless, it also slowed the rate of ATP depletion during the episode of sustained ischemia, so that after 10 minutes of ischemia, preconditioned hearts had more ATP than controls. However, after 40 minutes, ATP contents were not significantly different between groups. Preservation of ATP resulted from reduced ATP utilization and was not due to increased ATP production. Accumulation of purine nucleosides and bases (products of adenine nucleotide degradation) was limited in preconditioned myocardium. Accumulation of glucose-1-phosphate, glucose-6-phosphate, and lactate also was reduced markedly by preconditioning, due to reduced rates of glycogen breakdown and and anaerobic glycolysis. We propose that preconditioning reduces myocardial energy demand during ischemia, which results in a reduced rate of high energy phosphate utilization and a reduced rate of anaerobic glycolysis. Either preservation of ATP or reduction of the cellular load of catabolites may be responsible for delaying ischemic cell death.