A study was made of sodium efflux from squid giant axon, and its sensitivity to external K and Na. When sodium efflux from untreated axons was strongly stimulated by K(o), Na(o) was inhibitory; when dependence on K(o) was low, Na(o) had a stimulatory effect. Incipient CN poisoning or apyrase injection, which produces high intracellular levels of ADP(1) and P(i), rendered sodium efflux less depe...