On incubation with the spin trap alpha-(4-pyridyl-l-oxide)-N-tert-butylnitrone (4-POBN), a characteristic electron spin resonance (ESR) signal was produced at a greater rate in hepatic microsomal fractions from malignant hyprthermia susceptible (MHS) pigs compared with resistant (MHR) pigs. This was accompanied by increased formation of thiobarbituric acid reactive substances (TBARS). Supplementation of diets for six weeks with 235 mg alpha-tocopherol acetate/kg significantly increased microsomal vitamin E content of both pigs types. Moreover, the rate of formation of TBARS and ESR signal height of incubated microsomes from supplemented MHS pigs was decreased to that of MHR pigs. Elevated pyruvate kinase activities and TBARS concentrations in plasma of MHS pigs were also moderated by dietary vitamin E. Vitamin E supplementation may decrease the peroxidative events associated with MH.