Evidence is presented in support of the view that during the initial phase (lasting 1 to 3 hours) of Rb+ absorption by low-salt barley roots the root symplast comes close to equilibrium with the external Rb + solution. The characteristics of this state of equilibrium are: 1. proportionality between the equilibrium content and the initial rate of entry into the symplast; 2. reduction of influx which increases with the degree of saturation of the transport mechanism; 3. a lower degree of specificity in the internal control of influx than in external uptake; 4. temporary reduction of influx after a change of external conditions (from A to B) leading to a lower absorption rate, below the value prevailing after equal exposure to conditions B. These results are discussed on the basis of a tetrameric transport model in which there is an interplay of movement of ions in the free and in the bound state but in which no special feedback mechanisms are operating.