The difference in electrical potential between the xylem exudate of isolated roots of 1-week-old maize seedlings and the culture solution surrounding the roots has been determined together with the concentrations of potassium, calcium, chloride, and sulphate in the xylem sap. The difference in electro-chemical potential (Δμ) for these ions has been calculated from the measurements. The effects on Δμ of varying the salt status of the roots, the composition of the culture solution and of 2-4-dinitrophenol have been examined. Δμ for potassium and chloride was always positive, implying that movement of these ions to the xylem sap was under metabolic constraint. However, pretreatment of the maize seedlings with potassium chloride and increasing levels of dinitrophenol in the culture solution over the exudation period caused little or no significant change in Δμ for potassium although the rate of movement of ions to the xylem was substantially reduced. For calcium, Δμ was negative with roots in dilute culture solutions in the absence of dinitrophenol, implying that calcium could enter the xylem by passive diffusion. Addition of dinitrophenol changed the sign of Δμ and thus brought about an apparently active transport of calcium.Only in the absence of competing anions was the rate of entry of chloride significantly correlated with Δμ, and no such relationship was found for potassium or calcium individually. These results encourage doubt as to whether measurements of Δμ provide a valid basis for deciding to what extent the movement of individual ions depends on specific ‘active’ or ‘passive’ transport processes.The difference in electrical potential appears to be a characteristic of the living root and to depend on the total concentration of ions in the external solution and on their rate of transfer into the xylem sap.