Studies have been carried out on the movement of salt and water across the small intestine of the rat. Segments of the ileum of anesthetized rats have been perfused i~ v/vo with unbuffered NaC1 solutions or isotonic solutions of NaC1 and mannitol. Kinetic analysis of movements of Na ~ and CP s has permitted determination of the eCflux and influx of Na and C1. Net water absorption has been measured using hemoglobin as a reference substance.water was found to move freely in response to gradients of osmotic pressure. Net water flux from isotonic solutions with varying NaC1 concentration was directly dependent on net solute flux. The amount of water absorbed was equivalent to the amount required to maintain the absorbed solute at isotonic concentration. These results have been interpreted as indicating that water movement is a passive process depending on gradients of water activity and on the rate of absorption of solute.The effiuxes of Na and Cl are linear functions of concentration in the lumen, but both ions are actively transported by the ileum according to the criterion of Ussing (Acta Physiol. ScarM., 1949, 19, 43). The electrical potential difference between the lumen and plasma has been interpreted as a diffusion potential slightly modified by the excess of active C1 flux over active Na flux.The physical properties of the epithelial membrane indicate that it is equivalent to a membrane having negatively charged uniform right circular pores of 36/~ radius occupying 0.001 per cent of the surface area. I t has long been recognized that the intestinal epithelium is permeable in both directions to dissolved substances and to water. Goldschmidt and Dayton (1) showed that NaC1 entered the colon of dogs from the blOod, and a number of investigators have observed that water moves relatively freely, since it is absorbed rapidly from hypotonic solutions placed in the intestine but enters hypertonic solutions. Visscher et al. (2,3) using radioactive Na and C1 and D~O as tracers confirmed and extended these earlier results. More recently, absorption of NaC1 and water has been studied