Discharge patterns and CO₂-responsiveness of the efferent respiratory activities in the superior laryngeal (Xsl), hypoglossal (XII), and phrenic (Phr) nerves were compared between vagi-intact and -denervated rats. Bilateral cervical vagotomy decreased the respiratory frequency (f), minute Phr activity (peak integrated Phr activity × f) and consequently elevated end-tidal P_co2 (PET_co2). Augmentation of the peak inspiratory activity following vagotomy was much larger in the Xsl and XII nerves than that in the Phr nerve. After vagotomy, the time delay from the onset of inspiratory activities in the Xsl or XII nerve to the onset of the Phr bursts was greatly prolonged. While in the vagi-intact rats the peak inspiratory activities of these cranial nerves were not increased in response to elevated PET_co2, the activities, in particular of the XII nerve, were augmented by high PET_co2 in the absence of vagal afferents. These results suggested that the vagal afferents inhibit not only phasic Phr discharges but also inspiratory output neurons in the Xsl or XII motor nuclei being activated in normo- and hypercapnia and that they facilitate temporally the onset and progress of inspiratory activities in various groups of respiratory output neurons.