We have previously reported that the prolonged transient acidosis during early reperfusion mediates the cardioprotective effects in canine hearts. Recently postconditioning has been shown to be one of novel strategies to mediate cardioprotection. We tested the contribution of the prolonged transient acidosis to the cardioprotection of postconditioning. Open-chest anesthetized dogs subjected to 90 minutes occlusion of left anterior descending coronary artery (LAD) and 6 hours reperfusion were divided into four groups; 1) Control Group; no intervention after reperfusion (n=6), 2) Postcon Group; 4 cycles of 1 minute reperfusion and 1 minute re-occlusion (n=7), 3) Postcon+sodium bicarbonate (NaHCO₃) Group; 4 cycles of 1 minute reperfusion and 1 minute re-occlusion with the administration of NaHCO₃ (n=8) and 4) NaHCO₃ Group; an administration of NaHCO₃ without postconditioning (n=6). Infarct size, the area at risk (AAR), collateral blood flow during ischemia and pH in coronary venous blood were measured. The phosphorylation of Akt and extracellular signal-regulated kinase (ERK) in ischemic myocardium was assessed by Western blot analysis. Systemic hemodynamic parameters, AAR and collateral blood flow were not different among 4 groups. Postconditioning induced the prolonged transient acidosis during early reperfusion phase. An administration of NaHCO₃ completely abolished the infarct size-limiting effects of postconditioning. Furthermore, the phosphorylation of Akt and ERK in ischemic myocardium induced by postconditioning were also blunted by the co-treatment of NaHCO₃. In conclusion, postconditioning mediates its cardioprotective effects possibly via prolonged transient acidosis during early reperfusion phase with the activation of Akt and ERK.