In a chemical mutagenesis screen, we identified the novel Scn8a^8J allele of the gene encoding the neuronal voltage-gated sodium channel Na _v 1.6. The missense mutation V929F in this allele alters an evolutionarily conserved residue in the pore loop of domain 2 of Na _v 1.6. Electroencephalography (EEG) revealed well-defined spike-wave discharges (SWD), the hallmark of absence epilepsy, in Scn8a^8J heterozygotes and in heterozygotes for two classical Scn8a alleles, Scn8a^med (null) and Scn8a^med-jo (missense). Mouse strain background had a significant effect on SWD, with mutants on the C3HeB/FeJ strain showing a higher incidence than on C57BL/6J. The abnormal EEG patterns in heterozygous mutant mice and the influence of genetic background on SWD make SCN8A an attractive candidate gene for common human absence epilepsy, a genetically complex disorder.