Excised barley roots were allowed to absorb Rb and Cs from single or mixed Rb- and Cs-chloride solutions of various concentrations in the presence of 10 me/1 CaSO-j. Absorption periods of 3 and 6 hours respectively enabled the separate estimation of the steady rate of uptake setting in after about 3 hours and the additional amount of cation absorbed within the first 3 hours, which is supposed to represent cation bound to protoplasmatic binding sites. The following results were obtained. 1. The absorption and competition behaviour of Rb and Cs at different concentrations cannot be understood on the basis of the view that the binding sites involved in the absorption of these ions are separate and completely independent units. 2. In some important features there is a close though not absolute similarity between the effect of concentration on the steady rate of absorption of Rb and Cs after 3 hours, on the one hand, and on the additional amount of Rb and Cs absorbed within the first 3 hours, on the other. The bearing of these results on our notions concerning the nature of the carrier mechanism is discussed, and a structural conception of the association between ion and carrier advocated.