The kinetics of movement of tracer Na into human and dog red cells have been studied. The time courses of these processes and of K transfer were compared with the theoretical time course for saturation of a flat sheet having a resistive surface. The theoretical and the experimental curves when plotted against t((1/2)) have a considerable portion which is linear; on the basis of this resemblance the results are interpreted in terms of a permeability constant and an internal diffusion constant. It is supposed that selective adsorption acts to bring about concentration of K in the human cell and that the bulk of the Na of that cell is present in a thin outer region, while the K is in the interior. The action of strophanthin is to remove the usual limit to the Na capacity of the cell and it is proposed that the Na region increases in thickness at the expense of the K region. Omission of K from the medium has a similar result. Na uptake into poisoned cells measured either with tracer or as a net gain has a linear dependence upon t((1/2)) after a delay. The permeability of the dog cell to Na is reduced when K is added to the medium; this may be due to the formation of an outer K-rich region which imposes a resistance to Na movement.