Membrane potentials and vacuolar potassium activities of cells in the tissues likely to be involved in the movement of ions to the xylem vessels of maize roots have been measured. From this information it has been possible to build up a detailed picture of the profile electrochemical potential across the roots. For roots bathed in 1.0 mM KCl solution the results indicate an active movement of potassium to the vacuoles. The most striking finding was that this ability to accumulate potassium actively was the same for the cells of all the tissues investigated. Consequently there appeared to be no marked gradient in the driving force on potassium from cell to cell across the maize root under the conditions of our experiments. In view of the uniformity in the membrane transport properties of all the cells, it suggested that the endodermis does not have any special secretory role in the radial movement of potassium ions. It is concluded that the results are consistent with the symplasm theory of ion movement. However, contrary to the results of some workers, they indicate that the living stelar cells are no more leaky than those of the cortex.