Experiments are described in which it is shown that exchangeable rubidium ions in pea roots are transported to the shoot. The rate of this translocation is dependent on the transpiration rate. The exchangeable ions are released by the tissue to the xylem vessels without leaching out towards the medium. It is suggested that this Indicates the presence of an ion barrier in the plasmalemma. The apparent free space is thought to be localised at the root surface and in the cell walls of the cortex, whereas the exchangeable ions are found in the protoplasm.