1. The longitudinal distribution of cells capable of absorbing macro‐molecular material has been determined in the small intestine of the young rat by measurements of the uptake of [125I]polyvinyl pyrrolidone (PVP). 2. PVP uptake was never seen in the duodenum and became restricted to the distal half of the small intestine by the 18th day of life. The subsequent decline to zero in PVP uptake was due to an almost uniform decrease in PVP uptake by the distal half of the small intestine. 3. Histological studies indicate that this rapid decline in PVP uptake did not reflect a decrease in uptake by individual cells, but rather their replacement by new cells incapable of taking up PVP. 4. Autoradiographic estimates of turnover time in the intestinal epithelium using tritiated thymidine reveal good agreement between the time needed to replace completely the epithelium of the ileum (62 hr) and the duration of the rapid decline in PVP uptake (approximately 3 days). 5. It is concluded that on or about the 18th post‐natal day there is an abrupt change in the functional characteristics of the apical plasma membrane of cells produced by the crypts of Lieberkühn in the distal half of the small intestine. The stimulus for this change is unknown, but cells produced subsequently cannot take up PVP.