Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are found in drinking water and are metabolites of trichloroethylene. They are carcinogenic and promote liver tumors in B6C3F1 mice. Hypomethylation of DNA is a proposed nongenotoxic mechanism involved in carcinogenesis and tumor promotion. We determined the effect of DCA and TCA on the level of DNA methylation in mouse liver and tumors. Female B6C3F1 mice 15 days of age were administered 25 mg/kg N-methyl-N-nitrosourea and at 6 weeks started to receive 25 mmol/liter of either DCA or TCA in their drinking water until euthanized 44 weeks later. Other animals not administered MNU were euthanized after 11 days of exposure to either DCA or TCA. DNA was isolated from liver and tumors, and after hydrolysis 5-methylcytosine (5MeC) and the four bases were separated and quantitated by HPLC. In animals exposed to either DCA or TCA for 11 days but not 44 weeks, the level of 5MeC in DNA was decreased in the liver. 5MeC was also decreased in liver tumors from animals exposed to either chloroacetic acid. The level of 5MeC in TCA-promoted carcinomas appeared to be less than in adenomas. Termination of exposure to DCA, but not to TCA, resulted in an increase in the level of 5MeC in adenomas to the level found in noninvolved liver. Thus, hypomethylated DNA was found in DCA and TCA promoted liver tumors and the difference in the response of DNA methylation to termination of exposure appeared to support the hypothesis of different mechanisms for their carcinogenic activity.