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1. The dependency of the transmembranal potential differences upon the [Na] of the mucosal bathing solution was investigated in frog skin. Semilogarithmic linear correlations were obtained between [na]0 and the transepithelial potential difference (slope: 28-4 +/- 1-8 mV/dec [Na]0), the potential differences across the outer and the inner membrane under open circuit conditions (slope: 33-4 +/- 1-8 and 4-8 +/- 1-5 mV/dec [Na]0, respectively) and the intracellular potential under short circuit conditions (slope: 33-3 +/- 3-4 mV/dec [Na]0). 2. No difference could be observed between skins incubated in Cl- or SO4-Ringer. 3. The results are not in accordance with the Koefoed-Johnsen & Ussing hypothesis. Neither the polarity of the postulated Na electrode at the outer border could be obtained nor the expected slope of the dependency between potential gradient and [Na] of the mucosal bathing solution. 4. Current recirculation through paracellular shunt pathways is suggested to explain the deviation, at least in part. In addition, the I-R-drop resulting from Na entry across the outer border might account for part of the changes of potential gradients. 5. Under all conditions, Na uptake occurs energetically downhill, but permeability changes of the outer border must be postulated to explain the observed dependency of net Na transport upon [Na]0.