The effects of serotonin on jaw closer motoneurons (JCMs) were investigated by using intracellular recording in slice preparation in vitro. Recordings were made from JCMs in the brainstem slices of rats (3-6weeks old). Bath application of serotonin (20-100 microM) caused depolarization and increased input resistance in the most cells tested. Serotonin enhanced time-dependent inward rectification. These effects persisted in the presence of tetrodotoxin. In addition, serotonin increased firing frequency response to injected current in every cell tested. On the other hand, serotonin delayed the peak time of medium-duration hyperpolarizing spike afterpotential (mAHP) and increased the half decay time of the mAHP. These results suggest serotonin might play reciprocal roles on excitability of JCMs.