Low serum levels of vitamin D metabolites have been associated with an increased risk for colon cancer. To investigate the effects of vitamin D deficiency on the colon, 4-week-old mice were fed a diet either containing (100,000 IU/kg diet) or lacking this vitamin for 3 weeks. Food consumption and body weight gain were similar in both groups. Following injection with 3H-thymidine to label dividing cells, cellular proliferation and migration up the colonic crypt were determined autoradiographically. Although overall crypt lengths were similar in both groups, there was hyperplasia and hyperproliferation in crypts of the deficient animals. Also, their epithelial cells migrated up the crypt at a significantly slower rate (maximum 0.78 micron/h) than did those from control mice (1.42 microns/h). There was no difference in cellularity, proliferation or migration in duodenal epithelium. These results indicate that vitamin D deficiency significantly alters colonic but not duodenal epithelial cells.