Nephrotoxicity is one of the adverse side effects of methotrexate (MTX) chemotherapy. The mechanism of renotoxicity of MTX is not fully understood. It is essential to understand the mechanism of nephrotoxicity of MTX in order to diminish the side effects and hence maximize the benefits of chemotherapy. The aim of the study was to verify whether oxidative stress and neutrophil infiltration play a role in MTX-induced renal damage using a rat model. Adult male rats were administered MTX at the dose of 7 mg/kg body weight intraperitoneally for 3 consecutive days and sacrificed 12 or 24 h after the last dose. Vehicle-treated rats served as controls. The kidneys were removed and used for light microscopic and biochemical studies. Myeloperoxidase activity, a marker of neutrophil infiltration was measured in kidney homogenates along with the markers of oxidative damage including protein carbonyl content, protein thiol and malondialdehyde. The activities of the antioxidant enzymes, namely glutathione peroxidase, glutathione S-transferase, superoxide dismutase and catalase, were also assayed. MTX treatment induced damage to the glomeruli and tubules. Plasma creatinine levels in the MTX-treated rats were significantly elevated compared with controls. A significant increase in myeloperoxidase activity (p<0.05) was observed in the kidneys of MTX-treated rats. Protein carbonyl content and malondialdehyde, sensitive and reliable markers of oxidative damage to proteins and lipids, respectively, were significantly elevated (p<0.01) in the kidneys of MTX-treated rats compared with controls. The activities of the antioxidant enzymes, namely, superoxide dismutase and glutathione peroxidase, were significantly elevated (p<0.01 and p<0.05, respectively) in kidneys of rats following MTX treatment. The results of the present study provide evidence for the role of neutrophil infiltration and oxidative stress in MTX-induced renal damage. Administration of inhibitors of myeloperoxidase or scavenging hypochlorous acid, the product of myeloperoxidase, by supplementation with antioxidants as an adjuvant therapy may be promising in alleviating the renal side effect of MTX. Copyright 2009 S. Karger AG, Basel.