In plant cells, the Na(+)/H(+) antiport at the tonoplast provides a biochemical pathway to transport cytoplasmic Na(+) into the vacuole. Recently it was shown that overexpression of a vacuolar Na(+)/H(+) promotes sustained plant growth at high NaCl levels (Apse et al. Science 285, 1256, 1999). The sequestration of Na(+) ions into the vacuole can be followed using 31P and 23Na NMR spectroscopy. Suspension cell cultures are very suitable for this purpose and allow rapid and accurate assessment of the activity of the Na(+)/H(+) antiport and therefore potentially of salt tolerance. Perfusion experiments with maize cells that are not particularly salt (NaCl) tolerant showed that during salt stress the cytoplasmic pH remains unchanged while the vacuolar pH significantly increased. During Na(+) sequestration into the vacuole, the cytoplasmic pH equilibrates faster than that of the vacuole. Both vacuolar pH and the cellular Na(+) uptake rate were dependent on extracellular Na(+) for concentrations up to approximately 300 mM. For Na(+) concentrations >/=300 mM, both vacuolar pH and cellular Na(+) uptake became independent of the extracellular concentration. This indicates either a saturation of Na(+) uptake at the cell surface or a saturation of the Na(+)/H(+) transporter at the tonoplast. Na(+) uptake into the cell is accompanied by a rapid increase in vacuolar PO(4)(3-), broadening of the 31P resonances and a reduction in glucose monophosphate and UDPG.