The electrical potential difference (PD) across the membrane of the Ehrlich mouse ascites tumor cell has been measured with iniracellular microelectrodes. The mean for 111 cells in control Ringer solution was -11.2 m V ? 0.29 (SE), interior negative. When sulfate replaced chloride in the external medium the PD fell to -2.8 mV if measured as soon as possible after mixing the cells with a sulfate medium, but when nitrate replaced chloride the PD fell only to -8.5 mV. Cells equilibrated in nitrate had the same PD as those in control Ringer. These results indicate that the PD is sensitive to changes in the external chloride concentration and that nitrate can substitute for chloride electrically. However, since the PD for chloride, based on the Nernst equation and calculated on the basis of 70% exchangeability of cell chloride, is three times greater than the measured PD, it is hypothesized that sodium contributes significantly to the membrane potential in addition to chloride. On the other hand, potassium does not influence the PD to any great extent.

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Na+ transport was measured in the Ehrlich mouse ascites tumor cell by isotopic and chemical methods. Evidence for an initial, rapidly exchanging Na+ compartment was found in addition to a slower compartment. Net Na+ efflux might constitute less than half the total efflux. Influx of Na+ decreased while intracellular Na+ was falling, demonstrating a dependence of Na+ influx on intracellular Na+ content. Taken together, the data provide evidence for a component of Na+ transport which is mediated but not active.

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