The acute effect of bile deprivation on colonic epithelial cell proliferation in Sprague-Dawley rats was investigated and compared with its effect on jejunum and ileum. 2. days after the creation of bile fistula, tritiated thymidine was injected and animals sacrificed 1 and 24 h later. Compared with control untreated animals, sham-operated restrained rats had a reduced labeling and mitotic index of the colonic epithelial cell population as well as a slower migration of cells to the lumen. Colonic cell proliferation in animals deprived of bile flow was reduced a further 50%. Moreover, no evidence of cell migration or appreciable decline in grain density was seen over 24 h in bile fistula rats. Alterations in cell proliferation in both sham and bile fistula treated rats became less marked as one proceeded proximally to the small bowel. Therefore, significant alterations in cell kinetics result when normal bile flow is interrupted, suggesting its importance in the regulatory control of colonic cell proliferation.