The influence of melatonin on the reduced glutathione content, activity of glutathione peroxidase, glutathione reductase, and glutathione transferase in blood serum, heart, liver, and skeletal muscle of rats under conditions of experimental rheumatoid arthritis development has been estimated. A change in these parameters toward normal control values under the action of hormone has been revealed. The results can be related to realization of the melatonin antioxidant and protective properties under conditions of oxidative stress accompanying the development of rheumatoid arthritis.