SUMMARY1. The diffusional permeabilities to24Na of collecting ducts and the ascending and descending limbs of loops of Henle were studied by microperfusion in isolated papilla of rat kidney.2. The mean permeability coefficients were 33.9 × 10−7m.s−1for descending limbs of the loop of Henle, 7.0 × 10−7m.s−1for ascending limb, and 4.0 × 10−7m.s−1for collecting ducts.3. Vasopressin did not cause any significant increase in the permeability of collecting ducts to sodium ions.4. The permeability coefficients are consistent with those required for a counter‐current system whose driving force is a sodium pump in the entire ascending limb of the loop of Henle. They suggest that solute enters the descending limb of the loop of Henle and is carried towards the tip of the papilla in the direction of fluid flow. The entry of sodium and urea into the descending limb would reduce markedly the efficiency of the proposed passive models for countercurrent multiplication and raises doubts as to the importance of such systems i