Phenobarbital (PB), a rodent non-genotoxic carcinogen, showed hormesis, biphasic effects on rat livercarcinogenesis. To test the hypothesis that the hormesis earlier observed for PB induced hepatocarcinogenesis mightalso exist in the TGF-α transgenic mice model, one which is highly susceptible to carcinogenesis, the carcinogenic orpromotion effects of a wide range of phenobarbital (PB) concentrations were investigated. Two weeks after a singlei.p. dose of 5 mg /kg bw of diethylnitrosamine (DEN) to 15 day old mice, animals were treated with diet containingPB at doses of 0, 2, 15 or 500 ppm. The incidence and multiplicity of tumors, including hepatocellular adenomas andcarcinomas, were significantly increased by the high dose of PB, but no significant difference among the groupsreceiving 2 and 15 ppm for liver tumors when compared to DEN alone group. The proliferating cell nuclear antigenindices for liver tumors and surrounding hepatocytes in high dose PB treated mice were significantly increased, butno change was noted at the lower doses. The total cytochrome P450 content in the liver was also elevated by 500 ppmof PB, while hepatic 8-OHdG levels demonstrated no significant change. In conclusion, PB at high dose enhancesDEN-induced hepatocarcinogenesis in TGF-α transgenic mice, but low doses lack any significant effects. One possiblemechanism of phenobarbital carcinogenicity might be influenced by cytochrome P450 system exhibiting a strongpromoting activity for liver of mice.