AbstractThe duodenal progenitor population has been investigated with tritium thymidine (H3‐T) and autoradiography in male Swiss albino mice to characterize the role that individual cell types (principal, goblet, and Paneth) have in the proliferative activity of the duodenal crypt of Lieberkühn.Incorporation of H3‐T into newly synthesized DNA of all three cell types was observed by one‐half after injection of the isotope. Labeled principal and goblet cells were distributed from the first cell of the cryptal mouth to the last cell at the cryptal base. The majority of labeled cells was found in the central region of the crypt Labeled Paneth cells were restricted to the last four cells at the cryptal base.Analysis of the percentage of labeled metaphase and mitotic figures after H3‐T administration showed that the progenitor cycle duration of principal and goblet cells was about 15 hours, while that of Paneth cells was approximately 80 hours. The mean S‐phase duration was 7.4, 7.4 and 8.7 hours for principal, goblet, and Paneth cells, respectively. Thus, the G1portion of the cell cycle was considerably longer for Paneth cells, while all phases of the progenitor cycle were of the same duration for principal and goblet cells.It appears from the data that the three cell types of the adult duodenal crypt comprise independent populations capable of self‐renewal. Our observations also suggest the existence of causal relationships between the positioning of the proliferative cells types in the duodenal microenvironment and their respective cyc