The Long-Evans with a cinnamon-like color (LEC) rat is a mutant of the Long-Evans strain that develops hereditary hepatitis and hepatoma with ageing. Age-related changes in the mRNA expression of DNA methyltransferase (DNA MTase) were examined in livers of LEC rats using Long Evans with an agouti color (LEA) rats as controls. A dramatic increase in the expression of this mRNA was observed in LEC rats at 20 weeks when acute hepatitis appeared. Their high mRNA levels were maintained until 52 weeks of age. The mRNA expression as well as DNA MTase activities were found to be higher in cancer lesions than in adjacent normal tissue. These increases may be related to liver regeneration and to early events in cellular transformation of LEC rats.