The polar transport of rubidium ions across Potamogeton leaves was studied in further detail by using labelled and unlabelled rubidium solutions. In the dark no net transport takes place, but there is a considerable exchange of rubidium ions across the leaf. On the other hand, in the light net transfer as well as a simultaneous exchange of the ions occur. A formula for the calculation of the rate of exchange in the presence of net transport was derived. Net transport and exchange may represent two independent parallel processes. If not, it must be concluded that the mechanism of this active transport is essentially different from the mechanism involved in the accumulation of ions into the vacuoles of plant cells.