The thymus hormones were reported to be effective on lipid peroxidation and the antioxidant system. Thymus plays a broader role than just regulating the immune system. Thymosin α₁ is the first subgroup extracted from thymosin F5 and has higher biological activity than thymosin F5. In the present study, we have examined the effects of thymosin α₁ on lipid levels and lipid peroxidation and glutathione (GSH) content in the plasma, liver and aorta tissues of atherosclerotic rabbits. At the end of thymosin α₁ treatment, we determined the lipid levels and lipid peroxidation of the plasma, liver and aorta tissues and hepatic subcellular fractions in these rabbits. Our results demonstrated that thymosin α₁ might normalize changed lipid levels and increased lipid peroxides and also elevate decreased GSH in the plasma, liver and aorta tissues of atherosclerotic rabbits. Results of this study suggest that thymosin a, may be beneficial to prevent and/or to treat atherosclerosis.