AbstractThe possibility of turnover of the epithelial duct cells was examined in the adult mouse pancreas by radioautography following continuous administration of3H‐thymidine for periods varying from 1 h to 60 days. One hour after an injection of3H‐thymidine, the label observed in small and large ducts was low but increased with the duration of the continuous infusion of3H‐thymidine and reached a level of about 67% cells labeled after 60 days. The rate of duct cell labeling was estimated from the regression line of the labeling index vs. time in four types of ducts classified according to their inner diameter and the presence of the adventitia and was given as 0.60% cells per day in small (adventitia‐free) ducts (π 4–12 μm), 0.89%, 1.02%, and 1.23% cells per day in large (adventitia‐including) ducts (π 15–29, 30–49, and 50–160 μm respectively). In contrast, the labeling index of aciner cells after a 60‐day infusion indicated an addition of only 0.02–0.07% per day, and that of islet cells 0.14–0.22% per day.It is known that most parenchymal cells belong to either expanding or renewing cell populations. The acinar cells of the pancreas have been shown to constitute an expanding population, a conclusion confirmed by the low addition of cells observed in the present work. However, the relatively high rate of cell addition in the duct epithelia indicates that they may turn over in a period of 2.7 months in the case of large ducts and 5.6 months in the case of small ducts. It is proposed that the added cells replace cells carried away by t