1. The uptake of orally administered [125I]polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) by the small intestine of young rats was determined 4 hr after feeding by measurement of the radioactivity remaining in the wall after flushing out the intestinal contents. 2. PVP uptake was proportional to the administered dose of PVP for doses of 1 mg and below. At higher doses, uptake was no longer in linear proportion, suggesting a degree of saturation of uptake. 3. Litters were reared in one of three environmental temperatures: 13, 20 and 30° C, and were tested to determine the age at which PVP uptake ceased (‘closure’). The results were evaluated statistically (see Appendix). 4. There was no significant difference between the time of closure in animals reared at 20 and 30° C, but, relative to these groups, closure was significantly delayed, by approximately 48 hr, in animals reared at 13° C. 5. Once closure had begun, there were no significant differences in the rate of decline in PVP uptake with age between animals reared at different environmental temperatures. 6. Separation of the young rats from their mother both before and after the test‐feed of PVP caused a considerable reduction in PVP uptake. Provided that the body temperature of the young rats was maintained, the main factor contributing to the reduction in PVP uptake appeared to be the absence of milk, rather than separation from the mother.