Experiments were conducted to determine if food intake and adrenalectomy influenced abnormal antioxidant defense mechanisms observed in obese mice. Paired male C57BL/6J mice of two genotypes, obese (ob/ob) and lean (+/?), were fed a nonpurified diet ad libitum or restricted (2.5 g/d) until 3 mo old. Obese mice had larger livers and kidneys but smaller brains than lean mice. Plasma ceruloplasmin activity of obese mice was 240% of that of lean mice. Restricting food intake but not adrenalectomy reduced this difference, but ceruloplasmin activity of obese mice was still 150% of that of restricted-fed lean mice. Glutathione peroxidase (GSH-Px) activity in liver of obese mice was 70% of that in control lean mice; however, in kidney GSH-Px activity was 135% of that in obese mice. Both liver and kidney GSH-Px differences were eliminated by food restriction but not by adrenalectomy. Blood and brain GSH-Px activity was not influenced by the mutation. Liver and kidney copper-zinc superoxide dismutase activity was lower in obese mice than in lean littermates, 30 and 20%, respectively. Food restriction eliminated this difference in liver but not in kidney. Glutathione S-transferase activity using 1-chloro-2,4-dinitrobenzene as substrate was 55% lower in liver (not kidney) of obese mice than in lean mice and this difference was not markedly influenced by food restriction. Obese mice have marked changes in the steady-state activities of a number of protective enzymes that are organ dependent and, in part, due to the hyperphagia associated with this mutation.