The main components of antioxidant enzyme system (AOS) are superoxid dismutase ( SOD) and glutathione reductase (GR) catalyses the conversion of the superoxide anion. The important role in AOS belongs to catalase and glutathione peroxidase which perform H2O2 to nontoxic products. Simultaneous determination of AOS activity and malonic dialdehide (MDA) concentration (the index of lipid peroxidation in tissues and blood) characterize cells complex resistance to damage factor. The Н. М. Кургалюк 58 ISSN 0201-8489 Фізіол. журн., 2001, Т. 47, № 2 effect of L-arginine, as a precusor of nitric oxide synthesis and blocator NO-synthase (Nw-nitro-L-arginine) on AOS of rats with different resistance to hypoxia under stress condition is unknown and were subject of our investigation. Experiments were done on liver and blood tissues of white laboratory rats. The experimental animals were divided on two groups depending on hypoxia factor: high resistance (HR) and low resistance (LR). The type of resistance was determined by the time of ability to respire in barocamera with oxygen deficient equal to 12.000 meters over sea level. The animals adaptation to laboratory conditions continue during 14 days after in barocamera presence. All animals were divided dependent on experiment conditions on fourth groups. The first group: intact (HR and LR) animals parentherally injected by 1 ml of 0.9 % NaCl solution. The second group was subject of stress condition. The third group: HR and LR animals injected parentherally by 1 ml L-arginine (Sigma, USA) dose (600mg /kg body weight). The fourth one: rats injected by 1 ml Nw-nitro-L-arginine (L-NNA, Sigma, USA)- the blocator of NO-synthase. The animals were decapitated 30 min after injection and stress condition under ethereal anesthesia. Activity of antioxidant system enzymes superoxid dismutase (SOD), catalase (CAT); glutathione reductase (GR), glutathione peroxidase (GP) were measured spectrophotometrically. Also was investigated the concentration of serum antioxidant ceruloplasmin (CP). Level of lipid peroxidation was estimate by examination of concentration of lipids of hydroperoxides (LHP) and malonic dialdehyde (MDA). Our data confirm suggesting that nitric oxide (NO) is a major regulator in the AOS enzymes activity and limit damage influence of AOF. Action precursor NO L-arginine might be capable of protective role in various disorders which are connected with hypoxia factor. Following thing can be interred the investigation of influence of nitric oxide adaptive answears in stress condition modelling of pathological processes in rats with different resistance to hypoxia and reflect the biological qualities data stady on AOZ and LP.