Consumption of excess methionine by rats is known to cause membrane damage, liver enlargement and accumulation of iron in the spleen. In this study two groups (n = 5) of male, Wistar rats were pair-fed either a methionine supplemented (20.0 g/kg) or control (2.0 g/kg) diet for 7 weeks. Hepatic and erythrocyte copper-zinc superoxide dismutase activities were significantly reduced (P less than 0.05 and P less than 0.001 respectively) by methionine supplementation while the activities of catalase (P less than 0.01 and 0.05) and glutathione peroxidase (P less than 0.05) were significantly increased. Methionine supplementation also increased hepatic lipid peroxidation (P less than 0.01), as measured by the level of thiobarbituric acid reactive substances, and iron (P less than 0.001) concentrations. These changes are indicative of increased oxidative stress resulting from methionine toxicity.