The extrusion mechanism for intracellular Mg2+ was investigated in voltage-clamped snail neurones using Mg2+-sensitive microelectrodes and mag-fura-2. The intracellular free magnesium ion concentration ([Mg2+]i) of snail neurones voltage clamped to -60 mV was estimated to be 0·57 ± 0·06 mM (mean ± S.E.M.; n = 12) using Mg2+-sensitive microelectrodes and 0·62 ± 0·05 mM (n = 15) using mag-fura-2 . Raising extracellular MgCl2 from 5 to 20 mM caused an average increase in [Mg2+]i of 0·25 ± 0·04 mM (n = 7). In three experiments, removing extracellular MgCl2 caused an average decrease in [Mg2+]i of 0·1 mM. Replacing extracellular Na+ with N-methyl-D-glucamine (NMDG) caused a rise in [Mg2+]i of 1·8 ± 0·5 mM (n = 7); [Mg2+]i recovered to resting levels when extracellular Na+ was restored. Iontophoretic injections of MgCl2 were used to raise [Mg2+]i. The rate of recovery from such increases in [Mg2+]i (calculated from the slope of the recovery) was inhibited by 85-100 % (n = 5) in the absence of extracellular Na2+ compared with control conditions. Raising extracellular Ca2+ from 7 to 35 mM caused a reversible rise in [Mg2+]i of 0·4 ± 0·05 mM (mean ± S.E.M., n = 7). It was concluded that in snail neurones the main mechanism for [Mg2+]i extrusion is a Na+-Mg2+ exchanger which may be partially inhibited be high extracellular Ca2+ concentrations.