Na+-H+ exchange and passive Na+ flux were investigated in cardiac sarcolemmal vesicles as a function of changing the ionic composition of the reaction media. The inclusion of EGTA in the reaction medium resulted in a potent stimulation of Na+ uptake by Na+-H+ exchange. It was found that millimolar concentrations of Mg2+ and Li+ were capable of inhibiting Na+-H+ exchange by 80%. One mechanism by which these ions may inhibit intravesicular Na+ accumulation by Na+-H+ exchange is via an increase in Na+ efflux. An examination of Na+ efflux kinetics from vesicles pre-loaded with Na+ revealed that Na+, Ca2+, Mg2+ and Li+ could stimulate Na+ efflux. Na+-H+ exchange was potently inhibited by an organic divalent cation, dimenthonium, which screens membrane surface charge. This would suggest that Na+-H+ exchange occurs in the diffuse double layer region of cardiac sarcolemma and this phenomenon is distinctly different from other Na+ transport processes. The results in this study indicate that in addition to a stimulation of Na+ efflux, the inhibitory effects of Mg2+, Ca2+ and Li+ on Na+-H+ exchange may also involve a charge dependent screening of Na+ interactions with the membrane.