Systemically administered ondansetron, a 5-HT3 receptor antagonist, reduces obstructive sleep-disordered breathing (OSDB) events in the English bulldog. The neural mechanisms through which ondansetron acts are unknown. 5-HT3 receptor immunoreactivity and mRNA have been detected in the vicinity of upper airway dilator motoneurons (UAWDMn's), suggesting that this receptor may contribute to the state-dependent modulation of UADMn's. To characterize 5-HT3 receptor activity within a representative UAWD nucleus, we performed acute microinjections of selective 5-HT3 drugs, 1-(m-Chlorophenyl)-biguanide HCl, an agonist, and ondansetron, an antagonist, into a major population of UADMn's, the hypoglossal nucleus (XII), in anesthetized, paralyzed and mechanically-ventilated rats. The 5-HT3-selective drugs neither altered the baseline XII nerve activity nor the excitatory effect of 5-HT microinjected into the XII. In contrast, systemic-administration of ondansetron (3 mg/kg) produced a significant increase in the inspiratory modulation of XII nerve activity (to 195.8%±19.3 of control, p3 receptors within the XII nucleus do not mediate 5-HT effects on XII motoneurons, rather antagonism of 5-HT3 receptors outside the XII nucleus can increase respiratory drive to XII motoneurons. These results highlight the importance of understanding serotonergic effects on respiratory drive outside the UAWD motor nuclei as we search for 5-HT drug therapies for OSDB.