Whole-cell patch-clamp experiments were performed with neurons cultured from rat dorsal root ganglia (DRG). Two types of Na+ currents were identified on the basis of sensitivity to tetrodotoxin. One type was blocked by 0.1 nM tetrodotoxin, while the other type was insensitive to 10 microM tetrodotoxin. The peak amplitude of the tetrodotoxin-insensitive Na+ current gradually decreased after depolarization of the membrane. The steady-state value of the peak amplitude was attained several minutes after the change of holding potential. Such a slow inactivation was not observed in tetrodotoxin-sensitive Na+ current. The slow inactivation of the tetrodotoxin-insensitive Na+ current was kinetically distinct from the ordinary short-time "steady-state" inactivation. The voltage dependence of the slow inactivation could be described by a sigmoidal function, and its time course had a double-exponential process. A decrease of external pH partially antagonized the slow inactivation, probably through an increased diffusion potential across the membrane. However, the slow inactivation was not due to change in surface negative charges, since a shift of the kinetic parameters along the voltage axis was not observed during the slow inactivation. Due to the slow inactivation, the inactivation curves for the tetrodotoxin-insensitive Na+ current were shifted in the negative direction as the prepulse duration was increased. Consequently, the window current activated at potentials close to the resting membrane potential was markedly reduced. Thus, the slow inactivation may be involved in the long-term regulation of the excitability of sensory neurons.