Voltage-gated sodium channels can be characterized by their sensitivity to inhibitors. Nav1.5 is sensitive to block by cadmium and extracellular QX-314, but relatively insensitive to tetrodotoxin and saxitoxin. Na v1.4 is tetrodotoxin- and saxitoxin-sensitive but resistant to cadmium and extracellular QX-314. Nav1.8 and Nav1.9 generate slowly inactivating (ITTXr-Slow) and persistent (I TTXr-Per) currents in sensory neurons that are tetrodotoxin- resistant. Tetrodotoxin sensitivity is largely determined by the identity of a single residue; tyrosine 401 in Nav1.4, cysteine 374 in Na v1.5 and serine 356 and 355 in Nav1.8 and Na v1.9. We asked whether Nav1.8 and Nav1.9 share other pharmacological properties as a result of this serine residue. I TTXr-Slow and ITTXr-Per were saxitoxin-resistant and resistant to internal QX-314. ITTXr-Slow was also resistant to external QX-314 and displayed a approximately fourfold higher sensitivity than ITTXr-Per to cadmium. The impact of the serine residue was investigated by replacing tyrosine 401 in Nav1.4 with serine (Y401S) or cysteine (Y401C). Both mutants were resistant to tetrodotoxin and saxitoxin. Whereas Nav1.4-Y401C displayed an increased sensitivity to cadmium and extracellular QX-314, the serine substitution did not alter the sensitivity of Nav1.4 to cadmium or QX-314. Our data indicates that while the serine residue determines the sensitivity of ITTXr-Slow and I TTXr-Per to tetrodotoxin and saxitoxin, it does not determine their insensitivity to QX-314 or their differential sensitivities to cadmium.