The sensitivity of voltage-dependent sodium current to the sodium channel blocker tetrodotoxin (TTX) is altered by transfection of a c-Ha-ras oncogene into an excitable cell line. Control AtT-20 cells, a cell line derived from a mouse anterior pituitary tumor, were found to express both a TTX-sensitive and a TTX-resistant sodium current. AtT-20 cells transfected with the c-Ha-ras gene expressed only a TTX-sensitive current. Properties of TTX-sensitive and -resistant currents were also examined. No differences in voltage dependence of activation or inactivation between the TTX-sensitive and -resistant currents were observed. The rate of inactivation of the TTX-resistant current in control cells was slower, than that of the TTX-sensitive current in either control or ras-transfected AtT-20 cells.