In the present study, we examined the role of the neurokinin-1 receptor (NK1R) in the modulation of respiratory rhythm in a functionally-identified bradypneic region of the ventral respiratory group (VRG) in the in situ arterially-perfused juvenile rat preparation. In electrophysiologically- and functionally-identified bradypneic sites corresponding to the Bötzinger complex (BötC), microinjection of the selective NK1R agonist [Sar⁹-Met(O₂)¹¹]-substance P (SSP) produced a significant reduction in phrenic frequency mediated exclusively by an increase in expiratory duration (T_E). The reduction was characterised by a significant increase in post-inspiratory (post-I) duration with no effect on either late-expiratory duration (E2) or inspiratory duration (T_I). In contrast, in a functionally-identified tachypneic region, corresponding to the preBötzinger complex (Pre-BötC), control microinjection of SSP elicited tachypnea. Pretreatment with the NK1R antagonist, CP99,994, in the BötC significantly attenuated the bradypneic response to SSP injection and blunted the increase in T_E duration. This effect of SSP mimicked the extension of T_E produced by activation of the Hering-Breuer reflex. Therefore, we hypothesised that activation of NK1Rs in the BötC is requisite for the expiratory-lengthening effect of the Hering-Breuer reflex. Unilateral electrical stimulation of the cervical vagus nerve produced bradypnea by exclusively extending T_E. Ipsilateral blockade of NK1Rs by CP99,994 following blockade of the contralateral BötC by the GABA_A receptor agonist muscimol, significantly reduced the extension of T_E produced by vagal stimulation. Results from the present study demonstrate that selective activation of NK1Rs in a functionally-identified bradypneic region of the VRG can depress respiratory frequency by selectively lengthening post-I duration and provide evidence that endogenous activation of NK1Rs in the BötC appears to be involved in the expiratory-lengthening effect of the Hering-Breuer reflex. In conclusion, our findings demonstrate that selective activation of NK1Rs in discrete regions of the VRG can exert functionally diverse effects on breathing.