Osmotic gradients of 150 mosmol/1 between intratubular fluid and blood in the kidney of Rana ridibunda induce a fluid movement down the osmotic gradient which is stronger into the tubule than out of it. In either case Na + is reabsorbed "dry' '.If the active Na+-transport is blocked by Furosemid an osmotically downhill movement of isotonic fluid out of the tubule obtains. Reversal of the osmotic gradient induces a reversal of flow but the inward moving fluid is diluted. The tubular cells seem to inhibit the backwash of Na+. If the osmotic gradient induces an outflow of fluid out of the tubule no influx of l~a+ occurs even in the presence of a concentration gradient for Na + in the opposite direction. The Na+-concentration in the urine falls considerably below that of the pcrfusion fluid. This finding indicates Na +-reabsorption against a concentration gradient. Upon blocking of the Na+-transport by Furosemid water enters the tubules.The results of the experiments in which the Na+-transport was blocked by Furosemid prove that the tubules are impermeable for Na+ in the direction of secretion. The "dilution" of the tubular fluid as a result of an osmotic gradient which induces an influx of water is not due to a reabsorption outbalancing "secretion".If instead of the easily penetrating NaC1 the slowly penetrating Na+-cyclohexanesulphamate is used, less Na + is transported. However, in every other respect the results obtained are similar to those described above.Zusammen]assung. Dutch artifizielle osmotische Druckdifferenzen yon 150 mOsmol/1 zwischen Lumen-und Blutseite der Tubulusepithelien yon Rana ridibunda wird Fliissigkeit im Richtungssirme der l~eabsorption oder --in geringerem MaBe --der Sekretion l~ngs des osmotischen Gradienten bewegt. Na + wird in jedem Fall ,,trocken" reabsorbiert.Bei mit Furoscmid blockicrtem aktiven Na+-Transport l~uft isoton Fliissigkeit l~ngs des osmotischen Gradienten aus dem Tubuluslumen in das blutseitige Medium. Bei umgekehrt gerichtetem Gradicnten wird das Filtrat so ver~ndert, als ob es dutch Einstrom reinen Wassers verdiinnt wiirde. Die Tubulusepithelien verhalten sich so, als bes~Ben sie --im Zustand des blockierten aktiven Na+-Transportes --ein ,,Riickschlagventil" fiir lga+. Wena der osmotische Gradient so gerichtet ist, dais Wasser oder Fliissigkeit aus dem Lumen in das blutseitige Medium l~uft, dalm