A potentiometric method to monitor cell volume changes in intact biological tissues is based on ion-selective microelectrodes filled with the classical K+ exchanger Corning 477317. which is very sensitive to large quaternary ammonium ions. After introduction into a single cell, these cations can be used as volume marker by relating their changes in concentration to changes in cell volume, provided that the ions are neither metabolized nor transported out of the cell. In this article we tested the applicability of this method in leech Retzius neurones. In addition to the cell volume, by using triple-barreled microelectrodes we also measured simultaneously the intracellular concentrations of physiologically relevant ion species as well as the membrane potential. We found that Retzius neurones possess an uptake mechanism which allowed for the intracellular accumulation of tetramethylammonium, tetraethylammonium or choline, simply by adding these ions to the bathing solution. In contrast, tetrapropylammonium was not taken up and therefore, we iontophoretically injected this ion species via conventional microelectrodes. The isoosmotic elevation of the extracellular K+ concentration from 4 to 30 mM (Na+ replaced by K+) induced a swelling of the cells. The changes in the intracellular concentrations of K+ and Na+ suggest that cell swelling is primarily due to the uptake of K+, probably with Cl- as counterion.