The mycotoxin ochratoxin A (OTA) is a widespread contaminant in human and animal food products. It induces a wide range of toxic effects including lipid peroxidation through the generation of free radicals. The aim of this work was to evaluate the antioxidant effects of melatonin against OTA-induced oxidative stress in liver and kidney in rats. Treated animals were fed OTA-contaminated diet (3 mg/kg) for 15 days before, during and after melatonin administration (20 mg/kg bw). The results indicate that OTA caused severe effects typical to those reported in the literature for ochratoxicosis. Melatonin alone was effective in the improving food intake, body weight gain, serum total protein, albumin, the activities of alkaline phosphatase, G-glutamyl transferase and creatinine kinase and liver and kidney glutathione peroxidase, superoxide dismutase and malondialdehyde. Rats fed OTA-contaminated diet before, during or after melatonin administration showed a significant improvement in all tested parameters toward the normal values of the controls. This improvement was most pronounced in the group pretreated with melatonin. It is concluded that melatonin exhibits a preventive effect against OTA-induced oxidative stress through its role in the scavenging of free radicals and/or the prevention of lipid peroxidation.