This study was designed to talk about the influence of the acute normovolemic hemodilution(ANH) in cerebral ischemia/reperfusion(IR) injury in terms of mitochondrial inner membrane. METHODS: 24healthy male SD rats were randomly divided into 4 groups(each one 6 rats):control group, ischemia/reperfusion group(IR), ANH Hct30% and ischemia/reperfusion group(HES30), ANH Hct40% and ischemia/reperfusion group(HES40). Ischmia was established by four-vessel cerebral ischemia/reperfusion model. Half an hour before carotid artery was blocked, blood was taken from the femoral vein, and in the meantime, the same amount of 6% hydroxyethyl starch(HES)solotion(130/0.4) was transfused. After 48h, the rats was executed and hippocampus was taken out. The activity of cytochrome c oxidase(CcO) were measured by spectro photometry instrument, and the expression of CcOII mRNA were determianted by RT-PCR method. Result:Compared with the negative control group, after cerebral/reperfusion, the activity of CcO decreased(p<0.01), the expression of CcO reduced(p<0.01). Application of 6%HES(130/0.4) in the pretreatment of the ANH in cerebral ischemia/reperfusion, increased CcO activity(p<0.01), and increased the expression of CcOII mRNA(p<0.01). When hemodilution to Hct 30%,it achieves the optimal outcome. Compared with the HES40 group, its CcO activity, CcOII mRNA expression were statistically significant(p<0.01,p<0.01). Conclusion:Application of 6%HES130/0.4 pretreatment of ANH can improve activity and expression of CcO in hippocampus mitochondria after cerebral IR. Its protection acieves the optimal outcome when hemodilution to Hct 30%. The appropriate acute hemodilution elevates themitochondrial function and energy supply,improving the outcome of the global cerebral ischemia/reperfusion.