A dialysis method for the study of intestinal absorption is described. Its use has been assessed in animals and normal human subjects and it has been applied to the measurement of rectal transport of sodium and water.When the luminal solution was of high sodium concentration (145 m-equiv/1), the sodium influx rate (lumen to plasma) was about five times greater than the sodium efflux rate (plasma to lumen). The luminal sodium concentration associated with zero net sodium flux was very low (<15 m-equiv/1). As the mucosa was charged with the luminal side negative, the epithelium must therefore possess a powerful sodium absorbing ;pump'. With isotonic solutions in the lumen, the amount of water absorbed depended on the sodium concentration and when this was 30 m-equiv/1 or less, no significant water absorption was detectable. When, however, water absorption was altered by imposing osmotic gradients, sodium absorption was not significantly affected. The luminal solution tended to become issomolar with plasma; osmotic gradients across the epithelium did not develop. The particular transport properties of rectal epithelium enabling it to remove sodium from the lumen against considerable electrochemical gradients are well adapted to its function.