Trans-root and membrane potentials have been measured simultaneously in the same maize root by using microelctrodes inserted in the exuding sap, external bathing solution, and a vacuole of an epidermal cell. On rapidly increasing the KCl concentration of the external solution, the membrane and trans-root potantials fell simultaneously. This initial rapid phase of depolarization was complete within 20s of changing the external solution when the membrane potential had reached a new stable value. However, the trans-root potential continued to fall slowly and this phase of depolarization lasted for about 25 min. Then followed a rise to a stable value at 1.5–2.0 h. This secondary rise was relatively small compared with the initial fall. The major part (approx. 80 per cent) of the depolarization of the trans-root potential occured during the initial rapid phase. These results indicate that the major component of the trans-root potential resides at the plasmalemma of the epidermal cells with a smaller contribution from the cells underlying the epidermis. The rise in the trans-root potential after 25 min suggested that this back potential was associated with the plasmalemma of the xylem parenchyma. From knowledge of the elelctrical properties of these cells this back potential could be calculated and trans-root potentials accurately predicted from values of the membrane potentials of root cells. It is concluded that in maize roots, ion movement to the xylem vessels is mainly symplasmic, that the outer boundery of the symplasm is the plasmalemma of the epidermal cells and that the inner boundary is the plasmalemma of the xylem parenchyma. This hypothesis has enabled trans-root electrochemical potential differences to be predicted accurately from vacuolar values.