This study investigates the effects of either a high-fat diet or endotoxin on peroxisome metabolism as assessed by measuring catalase activity, catalase mass, and peroxisomal β-oxidation. Three mouse strains C3H/HeJ, BALB/c, and C57BL/6J were fed either a low-fat or a high-fat diet and injected intraperitoneally with 1 microg Escherichia coli lipopolysaccharide. These parameters were not different in C3H/HeJ mice fed a high-fat diet compared with controls fed a low-fat diet. Total liver catalase activity and peroxisomal β-oxidation were higher in BALB/c mice fed high-fat diet compared with low-fat controls. Total liver catalase activity, catalase mass and peroxisomal β-oxidation increased to the greatest extent in C57BL/6J mice fed a high-fat diet. Endotoxin treatment did not alter any of the parameters in mice fed a low-fat diet. Among mice fed a high-fat diet, endotoxin did not affect hepatic catalase or peroxisomal β-oxidation in the C3H/HeJ mice, decreased catalase activity in BALB/c mice (28%), and greatly decreased both catalase (66%) and peroxisomal β-oxidation (69%) in C57BL/6J mice. The decrease in catalase activity in C57BL/6J mice was apparently because of an inactivation of the enzyme as determined by the activity/mass ratio. Thus, endotoxin is showed to inhibit both catalase and peroxisomal β-oxidation in mice and the sensitivity to endotoxin is greatest in C57BL/6J mice fed a high-fat diet.