The kinetics of T1 absorption by excised barley roots were studied with respect to the time and concentration dependence, sensitivity to Ca, and interaction with Rb and Na. The following conclusions were drawn: 1. T1 is bound by sites normally binding K. 2. In the range of concentrations from 0 to 0.2 me/l, Tl is absorbed readily into the protoplasm but vacuolar accumulation is slow and tends to stagnate. 3. Although 02 uptake is progressively reduced in the presence of Tl, this effect does not seem responsible for the stagnation of vacuolar absorption. 4. Besides a competitive inhibition, there is a stimulation of vacuolar Tl transport by Rb but not by Na. The peculiarities of Tl absorption are tentatively explained on the basis of structural interac- tions between a macromolecular carrier and the ions transported.