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1. The suitability of inulin and mannitol as markers of the diffusional space that exists between the surface of frog skin and the outer barrier to sodium transport has been compared in experiments designed to measure the rapid uptake of sodium.

2. Inulin and mannitol both equilibrate finally with the same space at the outer surface of the frog skin, but the rate at which this equilibration occurs is considerably faster for mannitol.

3. The dependence of the rapid uptake of sodium on the concentration of sodium in the external medium, using mannitol to correct for extracellular sodium, can be described by simple saturation kinetics with an apparent K_m for sodium entry of 24 mM and a maximal rate of uptake of 1·28 µequiv/cm². hr.

4. The effect of amiloride is to inhibit sodium uptake, the degree of inhibition depending both on the concentration of sodium in the external medium and on the level of transepithelial transport normally maintained by the skin.

5. Ouabain inhibits sodium uptake when the tissue has been pre-incubated in sodium containing medium. It has no effect on sodium uptake if the pre-incubation takes place in sodium free medium.

6. A short-circuit current and potential difference can be elicited across frog skin in the presence of ouabain, by increasing the concentration of sodium bathing the outside surface. This potential and short-circuit current is abolished by the presence of amiloride.

7. These results provide direct evidence that amiloride acts to stop entry of sodium into the transport compartment and suggest that the ouabain inhibition of sodium uptake is mediated primarily through an increase of epithelial sodium concentration.