1. A method has been developed which allows the quantitative estimation of the uptake of labelled polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) by the wall of the small intestine of young rats. 2. Four hours after feeding a standard dose of [125I]PVP by stomach tube, the small intestine was thoroughly washed out, and the radioactivity of the intestinal wall measured. Under these conditions, the small intestine of animals less than 18 days old took up more than 50% of the radioactivity which had left the stomach. There was no increase in PVP uptake if the duration of absorption exceeded 4 hr. The PVP was taken up by the epithelial cells of the villus, and its intracellular localization has been demonstrated by fluorescence microscopy and can be related to vacuolation in the cells. 3. In animals between 18 and 20 days old the uptake of PVP declined progressively, until, in animals more than 20 days old, less than 5% of the radioactivity was taken up by the intestinal wall. 4. There is good agreement between the reported age of termination of antibody absorption in young rats and the age at which PVP uptake ceased in the present experiments. It is suggested that the loss of ability of the intestine to take up substances of high mol. wt. may be the factor which limits the duration of the period of antibody absorption in this species.