We investigated metabolism and carcinogenesis in livers of Sprague-Dawley rats fed juices and pelleted diets containing Korean native plants petasites japonicus Maxim by evaluating cell localization and expression of cytochrome P450s and GST-P. Anti-cytochrome P450s application in liver sections revealed three to four times increased expression of cytochrome P450E1 immunoreactivity in degenerative hepatocytes when compared to histologically normal hepatocytes. Anti-GST-P in showed positive pren plastic foci as well as in individual hepatocytes randomly scattered throughout all liver sections examined. Additionally GST-P was evident in proliferative endothelial cells and biliary epithelial cells in exposed rat livers. These results suggested that the increased level of cytochrome P4502E1 in affected hepatocytes was a direct consequence of Petasites japonicus toxicity. Further immunoreactivity to anti-GST-P in hepatocytes endothelial cells and biliary epithelial cells indicated a possible preneoplastic effects of Petasites japonicus in Sprague-Dawley rat.