In rats maintained on a carcinogenic diet (choline deficient containing 0.1% ethionine), the levels of c-myc and p53 mRNAs increased by 4 wk after animals were placed on the diet. Cell isolation studies showed that the change in c-myc takes place in oval cells, while p53 increases predominantly in oval cells but also in hepatocytes. To determine whether this increase is a consequence of cell proliferation or is associated with transformation, we have developed an in vitro model of hepatocarcinogenesis using epithelial cells isolated from the livers of rats fed the carcinogenic diet. When maintained in vitro with infrequent subculture, this cell line (LE/6) undergoes spontaneous transformation. Inoculation s.c. of the transformed cells into nude mice yields tumors histologically identified as hepatocellular carcinoma. We have used these cell lines to compare the cell cycle expression of c-myc and p53 mRNAs in untransformed, partially transformed, and tumorigenic LE/6 cells. We find that the expression of both genes is under cell cycle control in untransformed and partially transformed cells. However, complete transformation of this cell line is associated with constitutive expression of myc but not p53 transcripts. On the basis of this work we suggest that constitutive expression of c-myc may be a late event in hepatocarcinogenesis.