Two mutations in the sodium channel alpha subunit that have been implicated as the cause of periodic paralysis were studied by functional expression in a mammalian cell line. Both mutations disrupted inactivation without affecting the time course of the onset of the sodium current or the single-channel conductance. This is the same functional defect that was observed in myotubes cultured from affected patients and proves that these mutations are not benign polymorphisms. Unlike the currents in the myotubes, however, there was no consistent potassium dependence for the noninactivating component. These mutations also define new regions of the sodium channel alpha subunit that are involved in the process of inactivation.