In vitro culture of central nervous system neurons fromDrosophilalarvae enables direct examination of effects of neurological mutations at a single-cell level not readily amenable to in vivo experimentation. Using this system, we examined the cytotoxic effect of veratridine, which selectively causes persistent activation of sodium channels, on the mutantsparaandnapknown to have a temperature-dependent block in propagation of nerve action potentials. Even at a permissive temperature (22oC) for the mutant flies, the veratridine-induced neuronal lethality was significantly lower in bothparaandnapcultures than in normal cultures. At a temperature (35oC) causing paralysis of mutant flies,napneurons showed the same high degree of resistance to veratridine; whileparaneurons showed an increased resistance to a level similar to that ofnapneurons. A similar reduction in the veratridine-induced neuronal death was also observed in normal cultures that were pretreated with the sodium channel blocker tetrodotoxin. These results support the idea that both para andnapaffect sodium channel functions at the level of isolated single neurons. It was also found thatparaandnapmutations, like the sodium channel blocker tetrodotoxin, do not affect the morphological differentiation and survival of central nervous system neurons in culture. These findings indicate that functional sodium channels are not required for neurite outgrowth and survival of neurons at this developmental stage.