The quantity of sodium transported to the shoots of intact barley plants was stimulated by 0·5 mM ouabain when the sodium chloride level of the bathing medium was below 100 mM. At sodium chloride concentrations of 100 mM or more this ouabain-stimulated sodium transport was not observed. Equiosmotic mannitol, equimolar potassium chloride or equivalent calcium chloride solutions did-not affect the ouabain-stimulated sodium transport from a basic medium containing 10 mM sodium chloride. It is suggested that under the present experimental conditions the increased sodium uptake by the root cells at sodium chloride concentrations of 100 mM or more masks the extrusion process.