Mammalian heart sodium channels inserted into planar bilayers exhibit a distinctive subconductance state when single batrachotoxin-modified channels are exposed to external Zn2+. The current-voltage behavior of the open state and the Zn(2+)-induced substate was characterized in the presence of symmetrical Na+ ranging from 2 to 3000 mM. The unitary conductance of the open state follows a biphasic dependence on [Na+] that can be accounted for by a 3-barrier-2-site model of Na+ permeation that includes double occupancy and Na(+)-Na+ repulsion. The unitary conductance of the Zn2+ substate follows a monophasic dependence on [Na+] that can be explained by a similar 3-barrier-2-site model with low affinity for Na+ and single occupancy due to repulsive interaction with a Zn2+ ion bound near the external entrance to the pore. The apparent association rate of Zn2+ derived from dwell-time analysis of flickering events is strongly reduced as [Na+] is raised from 50 to 500 mM. The apparent dissociation rate of Zn2+ is also enhanced as [Na+] is increased. While not excluding surface charge effects, such behavior is consistent with two types of ion-ion interactions: 1) A competitive binding interaction between Zn2+ and Na+ due to mutual competition for high affinity sites in close proximity. 2) A noncompetitive, destabilizing interaction resulting from simultaneous occupancy by Zn2+ and Na+. The repulsive influence of Zn2+ on Na+ binding in the cardiac Na+ channel is similar to that which has been proposed to occur between Ca2+ and Na+ in structurally related calcium channels. Based on recent mutagenesis data, a schematic model of functionally important residues in the external cation binding sites of calcium channels and cardiac sodium channels is proposed. In this model, the Zn(2+)-induced subconductance state results from Zn2+ binding to a site in the external vestibule that is close to the entrance of the pore but does not occlude it.