Ingested 141Ce was retained to a much greater extent by suckling than by 26-day old or 43-day old rats. This greater retention was the result of a hold-up of 141Ce in the lower portions of the small intestine. Microautoradiographs showed 141Ce in these intestinal cells possibly associated with mucin vacuoles. The data suggest that 141Ce tends to remain in the epithelial cells until lost from the body by sloughing of the cells from the tips of the villi. The uptake and retention of cerium in rats is influenced by age and diet. Because of the concentration of 141Ce in the intestinal tissue of the suckling rat this radionuclide should be considered as more hazardous than previously regarded.