The electrical potential differences (Eex) and the electrochemical potential differences for potassium and chloride in the xylem exudate of excised maize roots were determined and the values obtained indicated active transport of both ions to the xylem exudate. The potential differences were depolarized by increases in the external KCl concentration ([KCl]o), and Eex was a linear fuction of log[KCl]o. The corresponding data for the cell vacuoles of the same roots were also determined and trends similar to those found for the exudate were obtained. The electrochemical potential differences between the cell vacuoles and the external solution for potassium (δμrac.k) and chloride (δμvac.Cl) indicated that both ions were actively transported to the vacuoles. Both δμvac.k and δμvac.cl decreased in a linear manner as [KCl]o was increased. However, in spite of the similar trends in both sets of data, the values of the electrical potential and chloride electrochemical potential differences were always markedly larger for the vacuoles than for the exudate. For potassium, there were no significant differences. It is concluded that the discrepancy between the data for the vacuoles and the whole roots does not support the idea that the exuding root system behaves like a single cell, as has been suggested by some workers.