The cellular concentrations of Na, K, and Cl have been measured in kidney slices of the amphibian, Necturus maculosus. Permeability coefficients have been determined for Na, K, Cl, Rb, Cs, and choline, from studies both of the uptake of radioactive isotopes and the rate of cell swelling in anisotonic solutions. The results of both methods were found to agree well. Measurements were also made of electrical potential differences across the peritubular face of the kidney cells using bathing solutions in which the electrolyte composition and concentrations could be varied. The data obtained are consistent with a model cell in which the potential difference arises as a result of differences in Na permeability relative to K on the two faces of the cell. The intracellular Na concentration is considered to be regulated by a Na-K coupled pump located at the peritubular face of the cell.

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