In some preparations the time constant of Na current inactivation determined with two pulses (tau c) is larger over some range of potentials than that determined from the current decay during a single pulse (tau h), while in others tau c(V) and tau h(V) are the same. Myxicola giant axons obtained from specimens collected from coastal waters of northeastern North America display a tau c - tau h difference under all conditions we have tested. In these axons tau c(V) and tau h(V) are unchanged by reduction of Na current density, addition of K-channel blockers, or internal perfusion. Specimens of the same species, Myxicola infundibulum, collected from a different geographical location, the south coast of England, have been studied under internal perfusion with K as the major cation internally, with reduced external Na concentration and in the presence of K-channel blockers. In these axons tau c(V) and tau h(V) approximately superpose, raising the possibility that dramatic differences in Na current kinetics may not necessarily reflect basic differences in the organization of the Na channel gating machinery.