['Gastrointestinal toxicity is one of the most serious side effects in the methotrexate (MTX) treatment. However, the mechanism of the toxicity has not been completely clarified, which may be the reason why symptomatic therapy is carried out. On the other hand, the oxidative stress is known to play an important role in various diseases and drug-induced side effects. In this study the focus was on the oxidative stress in order to clarify the mechanism of MTX-induced small intestinal damage, especially neutrophil infiltration.', 'MTX (20 mg/kg body wt) was administered to rats intravenously. Mucosal homogenates were prepared from the small intestine and used for assay of biochemical parameters, by which induction of oxidative stress and neutrophil infiltration were evaluated. N-acetylcysteine (NAC; 80 mg/kg body wt), an antioxidant or sodium tungstate (tungsten; 0.7 g/kg body wt), an inhibitor of xanthine dehydrogenase (XD)/xanthine oxidase (XO) known as an important source of reactive oxygen species (ROS) was given to rats with MTX to investigate the contribution of ROS to neutrophil infiltration.', 'The MTX treatment of rats induced the oxidative stress in the small intestine. The ROS production was seen preceding an increase of myeloperoxidase activity, which suggested neutrophil infiltration. Both treatments of NAC and tungsten prevented the MTX-induced ROS production and neutrophil infiltration.'] Does oxidative stress contribute to methotrexate-induced small intestinal toxicity in rats?