In vitro culture of central nervous system neurons from Drosophila larvae 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 mutants parats1 and napts known to have a temperature-dependent block in propagation of nerve action potentials. Even at a permissive temperature (22 degrees C) for the mutant flies, the veratridine-induced neuronal lethality was significantly lower in both parats1 and napts cultures than in normal cultures. At a temperature (35 degrees C) causing paralysis of mutant flies, napts neurons showed the same high degree of resistance to veratridine; while parats1 neurons showed an increased resistance to a level similar to that of napts neurons. 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 parats and napts affect sodium channel functions at the level of isolated single neurons. It was also found that parats1 and napts mutations, 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.