Multi-barrelled ion-sensitive microelectrodes were developed to enable simultaneous measurement of the free concentrations of up to three ion species and the reference potential in living cells. These electrodes were used for studies in Retzius neurones of the leech Hirudo medicinalis to investigate the effect of intracellular alkalinization on the release of Mg2+ from intracellular stores. The investigations were complemented by control measurements in micro-droplets containing ATP, which is presume to be a major intracellular Mg2+ buffer. The droplets were placed on a silver wire under paraffin oil and had approximately the same size (about 100 mu m in diameter) as the Retzius neurones. H+ or OH- were injected iontophoretically into living cells and micro-droplets from separate glass microelectrodes while the resulting changes in pH and Mg2+ concentration were recorded with a pH/Mg2+-sensitive microelectrode. Our results demonstrate that an alkalinization of the cytosol caused by an injection of OH- had no effect on the intracellular Mg2+ concentration ([Mg2+](i)) and thus did not change cytosolic Mg2+ buffering. When, however, alkalinization was elicited by the withdrawal of propionate from the bath solution, a method that also leads to the alkalinization of intracellular organelles, [Mg2+](i) increased, possibly by release of stored Mg2+ from mitochondria.