The general technique employed was identical with that of SOLS & PONZ, and phosphorus was determined by a modificarion of the FISKE & SUBBARROW's method. Experiments with the small intestine of rats, have shown that inorganic phosphate, ester phosphate and lipid phosphorus appeared int he intestinal mucosa during the absorption of glucose at different concentrations. However, protein phosphorus has never been found. Assays with arabinose showed also a secretion of phosphorus. Phosphorus do not appeard when solutions of 0.8% NaCl or 0.6 N glycine were used. If 0.8% NaCl was used after an experiment with hexoses or pentoses then a secretion of phosphorus was also observed. However, this secretion was smaller than the one obtained with sugar alone. The significance of these findings are discussed.