Changes in epithelial cell turnover play a central role in the regenerative response of the residual small intestine after partial small bowel resection. This article reviews these changes both in experimental animals and in man and discusses how alterations in cell turnover may influence villous morphology, the enzyme content of the cells and the absorptive and digestive function of the small bowel mucosa. Increased absorption per unit length of intestine has now been well documented after small bowel resection, and current evidence suggests that this is largely mediated by an increased absorptive surface produced by villus enlargement. In turn, the elongation of the villi seems to be due to hyperplasia with an increased number of cells rather than to hypertrophy of the individual cells. The villus hyperplasia is associated with an increased rate of cell migration although the cell turnover time may not be shorter than normal because the more rapidly migrating cells must traverse a greater distance before reaching the villus tips. Although absorption, per unit length of bowel, is increased after resection, the function of the individual cells appears normal or may actually diminish, presumably as a result of functional immaturity produced by more rapid cell migration from the nursery of the crypt to the adult world of the villus. Thus, when mucosal transport or cell enzyme content is expressed per mg of mucosal homogenate, per mg protein or per mg of DNA, there is either no change after resection, or the levels may even be reduced. The changes in cell turnover with the attendant effects on villous morphology, cell enzymes and absorptive function are all much greater in ileum than in jejunum. The mechanisms for the altered rate of cell turnover after resection are discussed. Intraluminal or topical nutrition, hormonal factors, the secretion of trophic substances in bile and pancreatic juice and changes in intestinal blood flow have all been proposed as factors responsible for the compensatory hyperplasia. It is suggested that studies of cell turnover after resection are of fundamental importance in understanding the controlling mechanisms of normal growth in the small intestine.