In voltage-gated Na+ channels the S6 transmembrane segment of domain IV (DIV-S6) is part of the lining of the inner part of the pore. It is of pivotal importance for inactivation gating. We recently showed that amino acid I1581 of DIV-S6 (rNaV1.4 amino acid numbering) is extraordinarily sensitive to both local and distal mutations suggesting a unique role in coupling of voltage-sensor movements to conformational changes in the pore. To date the only structural information relevant to voltage-gated Na+ channels can be derived from the recently crystallized bacterial channel NaVAb. In this structure the amino acid homologous to I1581 faces the lipid phase and is in close spacial relationship to the voltage-sensing apparatus. If this arrangement holds true for the eukaryotic Na+ channel then site 1581 should not be exposed to bulk solution.