Quaternary strychnine blocks sodium channels from the axoplasmic side, probably by insertion into the inner channel mouth. Block is strongly voltage dependent, being more pronounced in depolarized than in resting axons. Using potential steps as a means to modulate the level of block, we investigate strychnine effects on sodium and gating currents at +50 and -50 mV. We analyze our data in terms of the simplest possible model, wherein only an open channel may receive and retain a strychnine molecule. Our main findings are (a) block by strychnine and inactivation resemble each other and (b) block of sodium and gating currents by strychnine happen with closely similar time-courses. Our data support the hypothesis of Armstrong and Bezanilla (1977) wherein an endogenous blocking particle causes inactivation by inserting itself into the inner mouth of the sodium channel. Quaternary strychnine may act as an artificial substitute for the hypothetical endogenous blocking particle. Further, we suggest that at least 90% of the rapid asymmetrical displacement current in squid axons is sodium channel gating current, inasmuch as quaternary strychnine can block 90% of the displacement current simultaneously with sodium current.