AIM:To explore the characteristics of energy metabolism in brain mitochondria of rats exposed to acute and chronic hypoxia. METHODS: Animal grouping: Wistar rats were randomized into acute hypoxic group (AH), chronic hypoxic group (CH) and the control. Respiratory function, F 0F 1-ATPase activity, mitochondrial ATP, ADP and AMP contents and ATP production rate were measured respectively. RESULTS: In AH, brain mitochondrial respiratory state IV (ST 4) was increased, while respiratory control rate (RCR), mitochondrial ATP content, ATP production rate and F 0F 1-ATPase activity were decreased respectively. In CH, ST 4, RCR, mitochondrial ATP content and F 0F 1-ATPase activity were reversed partially. CONCLUSION: Acute hypoxia may impair brain mitochondria energy metabolism by way of depressing mitochondrial oxidative phosphorylation and ATP production and these parameters gain partial reablement during chronic hypoxia.