The ionic components of the action potentials of mouse spinal cord (SC) cells and dorsal root ganglion (DRG) cells were studied in dissociated cell cultures. It was found that the action potentials of SC cells required Na+ in the medium and were blocked by tetrodotoxin (TTX) (1 micron). Action potentials of DRG cells, on the other hand, were not blocked by TTX (up to 10 micron) and were observed in Na-free media in the presence of 8 mM Ca2+. In low Na (31 mM), low Ca2+ (0.1 mM) medium, action potentials were not observed but could be obtained if the Ca2+ concentration was increased. Action potentials of DRG cells investigated in low Na concentration in the presence of 1 mM or 8 mM Ca2+ became larger in amplitude and shorter in duration when the sodium concentration was increased. Na+ has this effect even in the presence of TTX. It is concluded that the action potentials of SC cells result mainly from a TTX-sensitive Na component. The action potentials of DRG cells on the other hand have both a TTX-insensitive Na component and a Ca2+ component.