The effects of oxygen inhalation for 48 h on the antioxidant capacity of lungs, livers, and brains in normal and vitamin E-deficient rats at various ages were examined. The activity levels of catalase, glutathione peroxidase, and superoxide dismutase, and the level of vitamin E in tissue homogenates were assayed as the indices of antioxidant capacity. Oxygen inhalation mostly decreased antioxidant enzyme activity in lungs. In particular, the catalase activity was much decreased. The glutathione peroxidase activity tended to be decreased. The superoxide dismutase activity was decreased in 32-month-old rats. Vitamin E deficiency did not augment oxidative damage due to oxygen inhalation. There appears to be no age effect on the oxygen-induced decrease in the antioxidant enzyme activities of lungs, except the superoxide dismutase activity in very old rats. Oxygen inhalation had some effects on the antioxidant capacity of livers and brains. For example, oxygen inhalation decreased the vitamin E concentration of livers in 32-month-old, normal rats. These results suggest that the antioxidant capacity of lungs is directly damaged by oxygen inhalation and that the antioxidant capacity of livers and brains is indirectly affected through lung damage. Antioxidant capacity may be maintained without large variation during young and middle ages, but its redundancy for emergency use may be diminished in old age.