The expression of the c-Ha-ras, the c-Ki-ras and the N-ras genes was measured by the dot blot technique in rat liver tumors induced by a short diethylnitrosamine (DENA) treatment and in the surrounding liver cells. A 2 to 25 times higher level of transcript was found as well in the surrounding cells, as in the tumor cells, as compared to the level in hepatocytes. In addition the increase of expression was parallel for the three ras genes. We conclude that this enhanced expression can be attributed to an epigenetic mechanism and it can, in certain cases, be dissociated from cell proliferation.