Whole-cell patch clamp recordings were performed on hypoglossal motoneurons in a brainstem slice preparation from the neonatal rat brain to study the characteristics of the fast Na+ current (INa) which has not been hitherto investigated in these cells. To aid voltage clamping of INa, cells were bathed in low Na+ solution, loaded intracellularly with Na+ (to reverse the Na+ gradient) or treated with a small dose (20 nm) of tetrodotoxin. In low extracellular Na+ solution (Na+ was replaced by choline or N-methyl-d-glucamine) INa activated at membrane potentials positive to −45 mV and was half-maximally activated at −30 mV. Similar data were obtained when the Na+ gradient was reversed or tetrodotoxin was applied. INa rapidly activated (1–3.5 ms time constant) and inactivated (1.6 ms time constant at 0 mV) during membrane depolarization. Inactivation was strongly voltage-dependent (half inactivation at −44 mV) and developed mono-exponentially. Recovery from inactivation was bi-exponential with fast and slow time constants of 14 and 160 ms, respectively, at −58 mV. The rapid turning on of INa was presumably responsible for the upstroke of the fast action potential generated by these cells while the slow phase of recovery from inactivation might modulate the ability to fire repetitively at high rate.