The modeling of acute immobilization stress caused the increase of the content of lipid peroxidation metabolites and the decrease of the activity of superoxiddismutase and catalase in liver tissue within 4 days after stress. Melatonin in dose 0.2 mg/kg had a weak antioxidant effect and increased superoxiddismutase activity. The injection of melatonin in dose 1.0 mg/kg caused the significant decrease of lipoperoxidation metabolites content in liver tissue 39 hours after stress. On fourth and seventh days of the experiment there were no differences in malonic dyaldehyde content between these animals and naive ones. The concentration of acylhydroperoxides was significantly less than in control and naive groups at these days. The activity of superoxiddismutase and catalase was significantly more than in control rats at the fourth day after stress.