Triple-barrelled, ion-sensitive microelectrodes were used to investigate the regulation of intracellular Mg2+ in Retzius neurones of the medicinal leech (Hirudo medicinalis). There is strong evidence that Mg2+ extrusion from Mg2+-loaded Retzius neurones is mediated by an amiloride-sensitive Na+/Mg2+-antiport, possibly working at a stoichiometry of 1:1. In contrast, Mg2+ influx pathways are more complex. Mg2+ influx could be partially inhibited by the Ca2+ channel blockers Ni2+, Co2+, La3+, and Gd3+. However, inhibition was especially prominent under conditions where the voltage-sensitive Ca2+ channels of Retzius neurones are closed, so that these results indicate the existence of specific Mg2+ influx pathways. For thermodynamic reasons a 1 Na+/1 Mg2+ antiport should reverse its mode of action in solutions with high Mg2+ content and contribute to Mg2+ influx, while a 2 or 3 Na+/1 Mg2+ antiport would still be able to extrude Mg2+ from the cell. As Mg2+ influx into Retzius neurones was Na+-independent and hardly affected by amiloride, a contribution of Na+/Mg2+-antiport, if present at all, is very small. However, an inhibitory effect of amiloride was detectable when, in addition, Mg2+ influx was reduced by the application of Ni2+ or Co2+. This reduction of Mg2+ influx by amiloride indicates a 1:1 stoichiometry of the Na+/Mg2+ antiport.