1. Roots of decapitated barley plants were allowed to equlibrate with Na+ solutions of different concentration. The kinetic characteristics of this process were examined. 2. Attainment of the saturation level appeared to involve not only strong reduction of the net flux but also of the influx as measured with 22Na + . 3. The final Na+ content of the tissue depended on the external Na+ concentration. Reckoned from a certain level of internal Na+ the relationship was the same as for the initial rate of Na+ uptake. 4. Transfer of roots saturated at a low Na+ concentration to a more concentrated Na+ solution brought about a resumption of Na+ absorption until a level characteristic of the new concentration had been attained. On the other hand transfer of the material from a higher to a lower concentration after saturation did not involve appreciable loss of Na+ from the tissue. 5. A kinetic model is proposed to explain these phenomena.