Changes in cell volume can be recorded by a potentiometric method based on ion-selective microelectrodes filled with the classical K+ exchanger Corning 477317, which is very sensitive to large quaternary ammonium ions (PNAS 82 (1985) 6014; Electrochim. Acta 47 (2001) 309). We investigated the effect of hyperosmotic bathing solutions on cell volume, the intracellular concentrations of Na+, Cl-, and K+ ([Na+](i), [Cl-](j), [K+](j)) as well as on the membrane potential of leech Retzius neurones by using multi-barrelled ion-selective microclectrodes. We found that an increase in the extracellular NaCl concentration induced a shrinkage of the cells as well as ail increase in [Na+](i), [Cl-](i) and [K (+)](i). Provided that the change in osmolality was moderate ( less than or equal to +40%), cell shrinkage and the changes in [Na+](i), [Cl-](i) and [K+](i) were proportional to the increase in the extracellular osmolality, i.e. the cells behaved like ideal osmometers. When the bath osmolality was increased more strongly, significant differences from the ideal behaviour were observed. In particular, the changes in [Na+](i) and [Cl-](i) were larger than expected from the changes in cell volume. The results show that leech neurotics raise their intracellular osmolyte content in response to severe osmotic stress, thereby reducing changes in their cellular volume. (C) 2003 Elsevier Science Ltd. All rights reserved.