To study the effects of dietry methionine level on lipid peroxidation of rats, rats were fed vitamin E, selenium and methionine - deficient diet or the same diet supplemented with various levels(0.3%, 0.6%, 0.9%) of methionine for 6 weeks. The biochemincal and mophological changes in the rat liver were investigated. Lipid peroxide levels in plasma and hepatic mitochondrial fraction of MF rats were more increased than those of control rats. However, supplementation with 0.6% methionine modified this increment. Catalase activity was decreased in hepatic mitochondrial fraction from rats fed MF diet. Methionine supplementation did not induce this enzyme. The ultrastructural evidence for lipid peroxidation was found in plasma membranes facing sinusoids. The most striking changes in including disruption and loss of microvilli and development of numerous lipid droplets occurred in rats fed MF diet. These changes were not effectively prevented by the same diet supplemented with 0.3% or 0.9% methionine, but supplementation with 0.6% methionine modulated more or less the changes.