Previous studies of red cell sodium transport have suggested two pumps. One is glycoside inhibitable and has been divided by Hoffman into pump IA and pump IB. Pump II is characterized by an additional component of sodium efflux which is inhibited by the addition of ethacrynic acid to maximally inhibiting concentrations of glycoside in the presence of external sodium. These studies have also suggested that the influence on sodium efflux conditioned by the presence or absence of potassium (ΔK) is independent of the concentration of sodium in the medium. These premises have been reevaluated, and it appears that so-called pump IB is determined by the nature of the counter ion used when sodium in the medium is deleted. When magnesium chloride is used in lieu of sodium chloride, pump IB is calculable, but when tetra-methyl-ammonium chloride is employed in lieu of sodium chloride, there is no pump IB. Furthermore, the △K. is very much dependent on the concentration of sodium in the medium. This is easily explained on the basis of the premise that the removal of potassium from the external medium depresses pump I, but permits the emergence of exchange diffusion provided external sodium is present. From these premises, the △K. can be calculated, and it is noted that this value does not differ from the observed. These observations are qualitatively the same in the erythrocytes from uremic patients with high concentrations of sodium.