1. Intestinal phosphate absorption in human subjects was studied by the technique of triple lumen intestinal perfusion in vivo.
2. Ileal phosphate absorption increased as the intraluminal phosphate concentration was increased.
3. Ileal rates of phosphate absorption were lower at any given intraluminal phosphate concentration than previously described jejunal rates. Acidification of the ileal lumen did not increase phosphate absorption.
4. Phosphate absorption was shown in the jejunum to be dependent on the intraluminal sodium concentration.
5. Phosphate absorption in the human small intestine consists of at least two components, one directly proportional to water movement and the second apparently independent of water movement.