Abstract Hepatic bile ductal cells are distinctively different in several morphological aspects from hepatocytes. Under a variety of conditions in which ductal cells proliferate in rat liver, this distinction remains, and cells possessing characteristics intermediate between the two were not seen in this study. Autoradiography with H 3TdR showed that the increased numbers of ductal cells arose by mitotic proliferation from pre-existing ductal cells and that hepatocytes arose from pre-existing hepatocytes. Furthermore, there was no evidence of significant transfer of label between the two types of cells. It is concluded that in the adult rat under the conditions studied each type of cell is specific and reproduces only its own kind.