The mechanism by which action potentials (APs) are generated in afferent nerve fibers from the carotid body is unknown, but is generally speculated to be caused by release of an excitatory transmitter and synaptic depolarizing events. However, previous results suggested that sodium (Na⁺) channels in the afferent nerve fibers play an important role in this process. To better understand the potential mechanism by which Na⁺ channels may generate APs, a mathematical model was constructed of chemoreceptor nerve fibers which incorporated Hodgkin/Huxley type Na+ channels with kinetics of activation and inactivation as determined previously from recordings of petrosal chemoreceptor neurons. While keeping the density of Na⁺ channels constant, spontaneous APs arose in nerve terminals as the axonal diameter was reduced to that present in rat carotid body. AP excitability and pattern were similar to that observed in chemoreceptor recordings: i) a random pattern at both low and high frequency discharge rates, ii) a high sensitivity to reductions in extracellular sodium concentration, and iii) a variation in excitability which increased with AP generation rate. Taken together, the results suggest that an endogenous process in chemoreceptor nerve terminals may underlie AP generation, a process independent of synaptic depolarizing events.