We studied the role of neurokinin-1 receptors (NK1-R) on the excitability of expiratory (E) neurons (tonic discharge, E_TONIC; augmenting, E_AUG; decrementing, E_DEC) throughout the ventral respiratory group (VRG), including Botzinger Complex (BotC) using extracellular single-unit recording combined with pressurized picoejection in decerebrate, arterially-perfused juvenile rats. Responses evoked by picoejection of the NK1-R agonist, [Sar⁹-Met(O₂)¹¹]-substance P (SSP) were determined before and after the selective NK1-R antagonist, CP99,994. SSP excited 20 of 35 expiratory neurons by increasing the number of action potentials per burst (+33.7±6.5% of control), burst duration (+20.6±7.9% of control) and peak firing frequency (+16.2±4.8% of control). Pretreatment with CP99,994 completely blocked SSP-evoked excitation in a subset of neurons tested, supporting the notion that SSP excitation was mediated through NK1-R activation. Since we had previously shown that E_AUG neurons were crucial to locomotor-respiratory coupling (LRC) (Potts et al. 2005), we reasoned that blockade of NK1-R would alter LRC by preventing somatic-evoked excitation of E_AUG neurons. Blockade of NK1-Rs by CP99,994 in the BotC severely disrupted LRC and prevented somatic-evoked excitation of E_AUG neurons. These findings demonstrate that LRC is dependent on endogenous SP release acting via NK1-Rs on E_AUG neurons of the BotC. Taken together with our earlier finding that inspiratory off-switching by the Hering-Breuer Reflex requires endogenous activation of NK1-Rs through activation of NK1-Rs on E_DEC neurons (Fong and Potts 2006), we suggest that endogenous release of substance P in the BotC provides a reflex pathway-dependent mechanism to selectively modulate respiratory rhythm.