Lamotrigine is a chemically novel anticonvulsant drug that has been reported to inhibit veratrine-induced neurotransmitter release from cortical slices in vitro. To characterize further the mechanism of action of lamotrigine, we have investigated the effects of this drug together with the anticonvulsant drugs phenytoin and carbamazepine on voltage-sensitive sodium channels present in N4TG1 mouse neuroblastoma clonal cells. Lamotrigine, phenytoin and carbamazepine produced a tonic inhibition of sodium channels with IC50 values of 91, 58 and 140 microM, respectively. At a concentration of 100 microM, all compounds shifted the voltage-dependency of steady-state inactivation toward more negative potentials by 7 to 15 mV, slowed the rate of recovery from inactivation and produced a use-dependent inhibition of sodium channels. Our data show that lamotrigine inhibits sodium channels in a manner that is similar to that produced by phenytoin and carbamazepine. This inhibition of neuronal activity is consistent with the reduction of glutamate release that was previously reported in neurochemical studies, and it expands our understanding of the mechanism of action of this anticonvulsant drug.