The B6C3F1 mouse is a hybrid strain which exhibits a high (30%) spontaneous hepatoma incidence and sensitivity to chemical induction of liver tumors. The spontaneous hepatoma incidence of the paternal C3H/He strain is approximately 60%, while that of the maternal C57BL/6 strain is very low. The presence of activated oncogenes, primarily Ha-ras, and to a lesser extent, Ki-ras, has been reported in B6C3F1 mouse liver tumors. Because alterations in a gene's capacity for expression, as well as mutation, may be involved in oncogene activation, this investigation was directed toward an examination of a putative control point for transcription, i.e., the methylation state of a gene. Hypomethylation is believed to be necessary, but not sufficient, for transcription. It was therefore hypothesized that alterations in the methylation state of the Ha-ras and Ki-ras oncogenes may facilitate the aberrant expression of these genes in B6C3F1 mouse liver. Restriction enzyme analysis (MspI, HpaII, and HhaI) was used to assess the extent of DNA methylation. MspI digestion of B6C3F1 and C3H/He DNA revealed the absence of a 15-kb Ha-ras band present in MspI-digested C57BL/6 DNA, suggesting that the Ha-ras oncogene of B6C3F1 and C3H/He mouse liver lacks a methylated site. In other respects, the Ha-ras and Ki-ras oncogenes are methylated to a degree which suggests that these oncogenes have a low potential for expression in normal mouse liver. The methylation state of the Ha-ras and Ki-ras oncogenes was also assessed in benzidine-induced hepatomas and adjacent nontumor tissue from B6C3F1 mice. In four out of four cases, the Ha-ras oncogene was hypomethylated in tumor as compared to nontumor tissue and increased expression of the gene was detected in three out of four hepatomas; the Ki-ras oncogene was hypomethylated in two out of four cases. These results suggest that hypomethylation of oncogenes may provide an epigenetic mechanism for facilitating their aberrant expression. The lack of a methylated site observed in the Ha-ras oncogene in B6C3F1 and C3H/He mouse liver may indicate an increased potential for its expression which could, in part, account for the high propensity for hepatoma development in these two strains.