Voltage-gated Na+ channels are responsible for the generation of the action potential. Hence functional defects of these channels can account for abnormal membrane excitability. In the central nervous system, these channels are composed of a pore-forming asubunit and two auxiliary subunits, b1 and b2. Mutations in a and b1 subunits have been linked to generalized epilepsy with febrile seizures plus (GEFS+), an inherited epilepsy syndrome. We studied the b1- and b2- mediated modulation of the brain Na_v1.1 channel in the absence and presence of a GEFS+-associated mutation, R1648H, using heterologous expression in tsA201 cells and the patch clamp technique. The b2 subunit had no detectable effect on channel gating, but b1 coexpression resulted in faster rates of inactivation and recovery from inactivation and a steeper voltage-dependence of activation for both WT and mutant channels. When the a subunit was expressed alone, the mutation resulted in a less steep voltage-dependence of steady-state activation and inactivation. Fast inactivation was slowed, recovery from fast inactivation accelerated and there was a significant non-inactivating persistent Na⁺ current. Coexpression of b1 and b2 subunits lowered most of the differences between WT and mutant channels. We conclude that b subunits refine the effects of the R1648H mutation, but modulate the gating of mutant and WT channels in a very similar manner.