Voltage-gated sodium channels (Na_V) are critical for initiation of action potentials. Heterozygous loss-of-function mutations in Na_V1.1 channels cause severe myoclonic epilepsy in infancy (SMEI). Homozygous null Scn1a^−/− mice developed ataxia and died on postnatal day (P) 15 but could be sustained to P17.5 with manual feeding. Heterozygous Scn1a^+/− mice had spontaneous seizures and sporadic deaths beginning after P21, with a notable dependence on genetic background. Loss of Na_V1.1 did not change voltage-dependent activation or inactivation of sodium channels in hippocampal neurons. The sodium current density was, however, substantially reduced in inhibitory interneurons of Scn1a^+/− and Scn1a^−/− mice but not in their excitatory pyramidal neurons. An immunocytochemical survey also showed a specific upregulation of Na_V1.3 channels in a subset of hippocampal interneurons. Our results indicate that reduced sodium currents in GABAergic inhibitory interneurons in Scn1a^+/− heterozygotes may cause the hyperexcitability that leads to epilepsy in patients with SMEI.