The role of acclimatization and the effect of persistent severe hypoxia (7000 m) were analyzed in mice soleus muscle with respect to oxidative stress (glutathione redox status) and damage markers (TBARS and SH protein groups), NAG and SOD activities and HSP70 expression. Forty mice were divided into one normobaric-normoxic control group and four hypobaric-hypoxic experimental groups (n = 8). One experimental group (1 D) was acutely exposed to a simulated altitude of 7000 m in a hypobaric chamber for 1 day. Another experimental group (ACCL + 1 D) was exposed to a 3 days acclimatization period plus 1 day of hypoxia exposure at 7000 m. The third experimental group (ACCL + 8 D) was exposed to the same acclimatization protocol, remaining 8 subsequent days at 7000 m. The fourth experimental group (8 D) was chronically exposed without acclimatization. ACCL + 1 D showed a significant decrease (p < 0.05) in oxidative stress and damage compared to the 1 D group. Concerning chronic severe hypoxia, acclimatization was truly vital, since 8 D animals died after 5 days of exposure. Oxidative stress and damage markers in ACCL + 8 D tended to gradually increase throughout the 8 days of the hypoxic period. Total SOD activity did not change in 1 D compared to control; however, it increased significantly (p < 0.05) in ACCL + 1 D and ACCL + 8 D. HSP70 expression followed the observed oxidative stress and damage pattern, suggesting a protective role against hypoxia-induced oxidative stress. The present study supports the hypothesis that acclimatization attenuates oxidative stress and damage induced by acute hypoxia, although a trend to a gradually increased oxidative deleterious effect in skeletal muscle seems to occur during persistent severe hypoxia even after a previous acclimatization period.