We expressed rat Nav1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in human embryonic kidney (HEK293) cells and evaluated the effects of the pyrethroid insecticides tefluthrin and deltamethrin on expressed sodium currents using the whole-cell patch clamp technique. Both pyrethroids produced concentration-dependent, resting modification of Nav1.6 channels, prolonging the kinetics of channel inactivation and deactivation to produce persistent “late” currents during depolarization and tail currents following repolarization. Both pyrethroids also produced concentration dependent hyperpolarizing shifts in the voltage dependence of channel activation and steady-state inactivation. Maximal shifts in activation, determined from the voltage dependence of the pyrethroid-induced late and tail currents, were ˜25 mV for tefluthrin and ˜20 mV for deltamethrin. The highest attainable concentrations of these compounds also caused shifts of ˜5–10 mV in the voltage dependence of steady-state inactivation. In addition to their effects on the voltage dependence of inactivation, both compounds caused concentration-dependent increases in the fraction of sodium current that was resistant to inactivation following strong depolarizing prepulses. We assessed the use-dependent effects of tefluthrin and deltamethrin on Nav1.6 channels by determining the effect of trains of 1 to 100 5-ms depolarizing prepulses at frequencies of 20 or 66.7 Hz on the extent of channel modification. Repetitive depolarization at either frequency increased modification by deltamethrin by ˜2.3-fold but had no effect on modification by tefluthrin. Tefluthrin and deltamethrin were equally potent as modifiers of Nav1.6 channels in HEK293 cells using the conditions producing maximal modification as the basis for comparison. These findings show that the actions of tefluthrin and deltamethrin of Nav1.6 channels in HEK293 cells differ from the effects of these compounds on Nav1.6 channels in Xenopus oocytes and more closely reflect the actions of pyrethroids on channels in their native neuronal environment.