Severe high-altitude hypoxia exposure is considered a triggering stimulus for redox disturbances at distinct levels of cellular organization. The effect of an in vivo acute and severe hypobaric hypoxic insult (48 h at a pressure equivalent to 8,500 m) on oxidative damage and respiratory function was analyzed in skeletal muscle mitochondria isolated from vitamin E-supplemented (60 mg/kg ip, 3 times/wk for 3 wk) and nonsupplemented mice. Forty male mice were randomly divided into four groups: control + placebo, hypoxia + placebo (H + P), control + vitamin E, and hypoxia + vitamin E. Significant increases in mitochondrial heat shock protein 60 expression and protein carbonyls group levels and decreases in aconitase activity and sulfhydryl group content were found in the H + P group when compared with the control + placebo group. Mitochondrial respiration was significantly impaired in animals from the H + P group, as demonstrated by decreased state 3 respiratory control ratio and ADP-to-oxygen ratio and by increased state 4 with both complex I- and II-linked substrates. Using malate + pyruvate as substrates, hypoxia decreased the respiratory rate in the presence of carbonyl cyanide m-chlorophenylhydrazone and also stimulated oligomycin-inhibited respiration. However, vitamin E treatment attenuated the effect of hypoxia on the mitochondrial levels of heat shock protein 60 and markers of oxidative stress. Vitamin E was also able to prevent most mitochondrial alterations induced by hypobaric hypoxia. In conclusion, hypobaric hypoxia increases mitochondrial oxidative stress while decreasing mitochondrial capacity for oxidative phosphorylation. Vitamin E was an effective preventive agent, which further supports the oxidative character of mitochondrial dysfunction induced by hypoxia.