5-Hydroxytryptamine (serotonin, 5-HT) affects upper airway and chest wall inspiratory muscle control. The purpose of this study was to investigate the relative interaction of serotonergic agents on these two muscle groups. We measured the responses of the hypoglossal and phrenic nerves to the systemic administration of serotonergic-active agents and determined the receptor types through which these agents act in anesthetized, vagotomized, paralyzed and artificially ventilated rats. The serotonin precursor, L-5-hydroxytryptophan (L-5-HTP) produced equivalent stimulation of phasic inspiratory activity of the hypoglossal and phrenic nerves. General serotonin antagonists produced significant and equivalent diminution of both motoneuron pools. Specific 5-HT1A stimulation and 5-HT1C/2 antagonism enhanced ventilatory activity. We conclude: (1) a baseline level of serotonergic input to hypoglossal and phrenic motoneuron pools was present, (2) different 5-HT receptors had different effects on ventilatory neural activity, and (3) hypoglossal and phrenic motoneuron pools responded similarly to the serotonergic agents given.