Experiments were conducted on 300 unbred male albino rats (weighing 150-200 g) under conditions of foothill plains (Frunze, 760 m above sea level) and the highlands (3,200 m above sea level) to study the antioxidant (AO) defence properties of the following enzymes: superoxide dismutase, catalase, glutathione peroxidase, as well as the low-molecular antioxidant glutathione. The activity of the defence enzymes and glutathione is marked by a complex dynamic, cyclic character with time. The effect of the AO enzymes is mutually dependent and is determined by the level of lipid peroxidation. By the end of one-month adaptation the glutathione concentration and activity of the defence enzymatic system functions on another steady level which differs from that at low-altitude.