The small intestinal epithelium is continuously renewed by proliferation of cells within the crypts, migration of these cells along the villus and their eventual extrusion at the villus tip. Epithelial cell migration and villus epithelial renewal rates are 4-5X slower in newborn than adult mice and increase progressively through the suckling period. To determine if developmental differences in the dynamics of epithelial renewal alter patterns of epithelial injury and repair, we have examined villus morphology and epithelial cell migration rates in newborn and adult mice following radiation injury. Proliferating cells were labeled with the thymidine analog, BrdUrd, either before or after exposure to 12 Gy gamma-irradiation and examined at intervals following injury using immunohistochemical staining. Epithelial cell migration rates at days of life 5, 8, 14 and adult were compared to age-matched, non-irradiated controls. Replicating cell populations in both adults and newborns decrease for the first 2 days after injury followed by regeneration of the proliferative epithelium. In adult mice the villi progressively shorten and are lost by day 4; however, in newborns the villus epithelium is maintained and villi do not shorten throughout injury-repair. Villus epithelial cells labeled before irradiation continue migration along the villus at normal rates in both adults and newborns. In contrast, regenerative epithelial cells produced 3 days after irradiation in adults migrate 40% slower than controls; while in newborn and suckling mice both cell migration and villus epithelial renewal rates accelerate following injury. From these studies we conclude that: (1) patterns of epithelial injury and repair are different in adult and newborn mice following radiation injury, and(2) the accelerated cell migration rates following irradiation may contribute to maintenance of the villi in newborn animals.