In skeletal muscle the activity of the enzymatic antioxidants superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) is regulated in response to generation of reactive oxygen species (ROS). Increased activity of these enzymes is observed after repeated bouts of aerobic exercise, a potent stimulus for intracellular ROS production. Hyperbaric oxygen (HBO) inhalation also stimulates intracellular ROS production although the effects of HBO on skeletal muscle SOD, GPx and CAT activity have not been studied. We tested the hypothesis that SOD, GPx and CAT activity is modulated in skeletal muscles in response to acute and repeated HBO administration. In adult male rats acute HBO inhalation (60 mm at 3 atmospheres absolute) reduced catalase activity by approximately 51% in slow-twitch soleus muscles. Additionally, repeated HBO inhalation (twice daily for 28 days) increased Mn2+-superoxide dismutase activity by approximately 241% in fast-twitch extensor digitorum longus muscles. We conclude that both acute and repeated HBO inhalation can alter enzymatic antioxidant activity in skeletal muscles.