The effect of acute hypobaric hypoxia against on the background of single-shot melatonin administration on the activity of glutathione ferments system (glutathione peroxidase, glutathione S-transferase and glucose-6-phosphate dehydrogenase) and the contents of reduced glutathione and malondialdehyde in the supernatant of juvenile male of white rats forebrain was investigated under three conditions of lighting--natural conditions of lighting in the spring-summer period of year, constant illumination and constant darkness during the one week. The constant illumination enhanced the lipid peroxidation with simultaneous decreasing in the activity of glutathione peroxidase in control animals. The constant illumination enhanced the lipid peroxidation with simultaneous decrease in the activity of glutathione peroxidase in control animals. The constant darkness reduced the intensity of free-radical oxidation improving the antioxidant neuron protection. The melatonin decreased the contents of malondialdehyde, raised the activity of glutathione enzyme system and eliminated the negative influence of constant illumination on forebrain antioxidant system in intact animals. The acute hypoxia resulted in the increase of lipid peroxidation and reduced the activity of antioxidant enzymes, which were most expressed in constant light conditions. The melatonin administration before of 30 minutes of acute hypoxia modeling reduced the intensity of oxidizing stress, which was generated by acute hypoxia.