The aim of this study was to identify the existence of anion-dependent Mg transport systems in cardiac muscle. DIDS-sensitive and anion-dependent (either Cl - o or NO - 3 o ) increases in [Mg 2 + ] i occurred during Mg 2 + loading conditions. Much larger elevations of [Mg 2 + ] i occurred under Cl - o -free conditions with 0.1 mmol l - 1 DIDS, compared to Cl - o replacement alone. All these effects were abolished in Mg 2 + o -free medium. These data suggest a novel Mg 2 + -anion symport for Mg 2 + efflux against the electrochemical gradient that is fueled mostly by the efflux of an endogenous anion (HCO - 3 ?), but with a small contribution from intracellular Cl - probably supplied via the Cl - -HCO - 3 exchanger.