SummaryAll the hepatocarcinogens administered to rats markedly increased the mitotic rate in the liver. Except for ɑ-benzene hexachloride and nitrosobutylurea, the nonhepatocarcinogens did not appreciably increase the mitotic rate. Cytogenetic analyses on the ploidy rate and chromosome abnormalities evidenced qualitative differences among mitotic liver cells in animals treated with hepatocarcinogens and nonhepatocarcinogens. The active cell proliferation and chromosome changes in liver cells observed after the administration of hepatocarcinogenic substances may have some relation to an essential process in hepatocarcinogenesis.