The aim of the present study was to examine the influence of intermittent hypoxia at rest and in combination with long-term high-intensity swimming exercise on lipid peroxidation and antioxidant defense system adaptation in skeletal muscles differing in fiber type composition. High-intensity chronic exercise was performed as swimming training with load that corresponded to ~ 75 % VO_2max (30 min·day^-1, 5 days·wk^-1, for 4 wk). Intermittent hypoxic training (IHT) consisted of repeated episodes of hypoxia (12%O₂, 15 min), interrupted by equal periods of recovery (5 sessions/day, for 2 wk). Sessions of IHT were used during the first two weeks and during the last two weeks of chronic exercise. Oxidative (red gastrocnemius and soleus, mix) and glycolytic (white gastrocnemius) muscles were sampled. Our results indicated that high-intensity swim training in combination with sessions of IHT induced more profound antioxidative adaptations in skeletal muscles than the exercise training only. This adaptation has muscle fiber type specificity and is reflected in significantly elevated superoxide dismutase and catalase activities in highly oxidative muscle only. Training adaptation of GSH system (reduced glutathione content, activities of glutathione reductase, glutathione peroxidase, NADPH-supplying enzyme glucose-6-phosphate dehydrogenase) occurred both in slow- and fast-twitch muscles. However, this process was more effective in oxidative muscles. IHT attenuated the increase in TBARS content induced by high-intensity swimming training. The test on exercise tolerance demonstrated a significant elevation of the swimming time to exhaustion after IHT at rest and after IHT in conjunction with high-intensity exercise in comparison with untrained and chronically exercised rats. These results confirmed that sessions of IHT might improve exercise tolerance and increase maximal work capacity.

• Single high-intensity exercise induces a significant increase in TBARS content, decreases in GPx, GR activities and GSH content in both fast- and slow-twitch muscles.

• Intermittent hypoxic training (IHT) may improve exercise tolerance and maximal work capacity.

• Antioxidant enzyme response to chronic exercise is highly muscle fiber specific.

• IHT induces no significant change in TBARS content in both slow- and fast-twitch muscles in comparison with normoxic rats.

• IHT promotes an adaptation of GSH system as well as antioxidant enzymes in skeletal muscle, however the biochemical mechanisms underlying the muscle fiber specific GSH adaptation to hypoxic training is not clear.