Excitatory responses to 5-hydroxytryptamine (5-HT) were recorded from the whole glossopharyngeal afferent nerve in anesthetized rats, using an electrophysiological technique. Bolus intravenous injection of 5-HT (6.25-50 micrograms/kg) evoked a dose-dependent excitation of glossopharyngeal afferent nerve activity with a rapid onset and lasting a few seconds. This response was blocked by a selective 5-HT3 receptor antagonist, YM060 (10 micrograms/kg i.v.). A 5-HT3 receptor agonist, 2-methyl-5-HT (6.25-50 micrograms/kg), also produced a rapid and dose-dependent excitation of this nerve activity, and YM060 (10 micrograms/kg i.v.) caused an inhibition of this effect. In contrast, efferent glossopharyngeal nerve activity was not altered by 5-HT even at a high dose (50 micrograms/kg). These results suggest that exogenous 5-HT may elicit excitation of the glossopharyngeal afferent nerve, and that this effect might be mediated via the 5-HT3 receptors on the nerve endings.