This study was investigated about the effect of onion (Allium cepa. Linn) on the chemical induction of preneoplastic lesions in rat liver. Male Sprague-Dawley rats were fed with control or onion powder diet for 9 weeks. Hepatocellular carcinogenesis was induced by a single intraperitoneal injection of diethylnitrosamine (DEN) (200 mg/kg BW) and 2/3 partial hepatectomy. Animals were sacrificed and livers were taken. Dietary supplementation of onion suppressed the formation of placental glutathione S-transferase (GST-P) positive foci in numbers (p<0.01) and area (p<0.05). Cytosolic activity of glutathione Stransferase (GST) was increased by DEN in the rats fed control diet. However, onion significantly decreased GST activity in DEN-treated rats. Glutathione peroxidase activity showed similar tendency to GST activity. Glutathione reductase activity and microsomal thiobarbituric acidreactive substance value, however, did not show noticeable difference among the groups. These results suggest that onion has anti-tumor activity that suppresses oxidative stress and the formation of preneoplastic foci in the rat liver.