It was investigated whether the movement of chloride ions through the osmotic barrier of Ehrlich cells is passive. i.e. driven by a membrane potential, which again is controlled by an active cation pump, or is active. Therefore, the distribution and the unidirectional fluxes of Cl−, along with the distribution of K+, Na+ and water between cells and medium, were measured while the cation pump was active (37°, Krebs‐Henseleit buffer, pH 7.4, 95% O2+ 5% CO2) and, for comparison, while the cation pump was inhibited by gassing with N2 instead of O2 or by adding DNP to the medium. In the presence of 1% bovine serum albumin, DNP altered the electrolyte levels inside the cell only at relatively high concentrations, 0.5–1 mM, then having the same effects as has anoxia. In either case, cells shrank by about 10%, attaining a new steady state which lasted for at least 20 min. K+ and Na+ gradients were than reduced to 2/3 and 1/2 of their normal values respectively. With cells thus shrunken and with control cells, exchange of cell chloride for labelled chloride in the medium reached an equilibrium after about 20 min. In the controls, the exchangeable fraction of total cell chloride, calculated
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