Myxicola giant axons internally injected with tetraethylammonium chloride to block potassium currents were examined under voltage clamp. The sodium inactivation time constants obtained from the decline in INa during step depolarizations were substantially smaller than those obtained using conditioning prepulses to the same potentials and the ratios agreed with previous observations using TTX. Inactivation shifts were also measured and found to be comparable to previous results.