Nonsteroidal anti-inflammatory drugs (NSAIDs) cause clinically important gastric damage by several mechanisms. In order to evaluate the role of neutrophil infiltration in lesion formation, tissue myeloperoxidase activities were assessed in different gastric layers of the stomach both in rats with normal neutrophil levels and in neutropenic rats. Sprague-Dawley rats were treated either with indomethacin (Indo; 25 mg/kg, s.c.) or the vehicle. A group of rats were made neutropenic by administration of methotrexate (MTX; 2.5 mg/kg i.p.) once a day for 3 days. The stomachs were removed for the determination of lesion index, glutathione, lipid peroxide levels, protein oxidation and tissue myeloperoxidase activities. MTX treatment appeared to reduce neutrophil infiltration significantly while producing insignificant effects on eosinophils and macrophages. Indo administration caused multiple gastric lesions and treatment with MTX significantly reduced lesion index. In rats treated with Indo, neither glutathione nor LP levels showed any significant changes but the protein oxidation was significantly higher than that of other groups. The MPO level of gastric mucosa was increased in Indo-treated rats and reversed by MTX pretreatment. The results of the present study indicate that neutrophil infiltration in the gastric mucosa of rats may be involved in the pathogenesis of NSAID-induced gastric mucosal injury, but no correlation was found between lesion formation and protein oxidation in the gastric mucosa.