Mg²⁺-extrusion from Mg²⁺-loaded neurons of the leech, Hirudo medicinalis, is mediated mainly by Na⁺/Mg²⁺ antiport. However, in a number of leech neurons Mg²⁺ is extruded in the nominal absence of extracellular Na⁺, indicating the existence of an additional, Na⁺-independent Mg²⁺ transport mechanism. This mechanism was investigated using electrophysiological and microfluorimetrical techniques The rate of Na⁺-independent Mg²⁺ extrusion from Mg²⁺-loaded leech neurons was found to be independent of extracellular Ca²⁺, K⁺, NO₃^-, HCO₃^-, SO₄^2-, HPO₄^2- and of the intra- and extracellular pH. Na⁺-independent Mg²⁺ extrusion was not inhibited by DIDS, furosemide, ouabain, vanadate, iodoacetate, 4-amino-hippurate or -cyano-4-hydroxycinnamate and was not influenced by changes in the membrane potential in voltage-clamp experiments. Na⁺-independent Mg²⁺ extrusion was, however, inhibited by the application of 2 mM probenecid, a blocker of organic anion transporters, suggesting that Mg²⁺ might be co-transported with organic anions. Extracellularly, of all organic anions tested (malate, citrate, lactate, -ketoglutarate, and 4-amino-hippurate) only high, but physiological, concentrations of malate (30 mM) had a significant inhibitory effect on Na⁺-independent Mg²⁺ extrusion. Intracellularly, iontophoretically injected malate, citrate or fura-2 but not Cl^-, -ketoglutarate, glutamate, succinate or urate were stimulating Na⁺-independent Mg²⁺ extrusion from those neurons that initially did not extrude Mg²⁺ in Na⁺-free solutions. Our data indicate that Mg²⁺ is co-transported with organic anions, preferably with malate, the predominant extracellular anion in the leech. The proposed model implies that under experimental condition malate drives Mg²⁺ extrusion, while under physiological conditions malate is actively taken up, driven by Mg²⁺, so that malate can be metabolized.