Adaptation in the small bowel after resection is associated with increases in crypt cell proliferation and villus height. This paper gives the results of an autoradiographic investigation with [3H]thymidine of epithelial cell migration 60 days after 40% small-bowel resection in the rat. The mean number of cell positions between the crypt--villus junction and the leading labeled cell 30 hr after injection was increased by 19.4% in the resected group (P less than 0.02). The mean total number of cells per villus column was increased by 27.8% (P less than 0.002). Migration rate estimated in cell positions per hour was accelerated by 18.9% (P less than 0.001) after resection. The 8.1% lengthened life span of villus cells was not statistically significant. The increased number of cells per villus column and unaltered life span of villus cells would facilitate functional adaptation. The causal relationship between the larger villus cell population and accelerated migration after resection and increased crypt cell proliferation is unknown.