We studied the levels of antioxidant and detoxifying enzymes in the livers and lungs of young and old rats kept under hypoxic or hyperoxic conditions as models of oxidative stress. In particular, we investigated the levels of enzymes directly involved in active oxygen species scavenging (superoxide dismutase, catalase and glutathione peroxidase-selenium dependent) and enzymes challenged with detoxification processes (glutathione transferase, glyoxalase I and glutathione reductase) in order to obtain a wide comparative view of the defence strategies used with respect to the age of the animals. The results show that the responses of some protective enzymes in young rats are opposite to those of old ones. Some of the changes found appear mainly due to age, while others appear to be due only to the oxygen tensions and are independent of the aging process. The glutathione contents of the liver and lung from young and old rats under hypoxic and hyperoxic conditions were measured.