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The present study explores how elevations in brain P increase the sympathetic nerve discharge (SND). SND, phrenic nerve discharge (PND) and putative sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVLM) were recorded in anaesthetized sino‐aortic denervated and vagotomized rats. Hypercapnia (end‐expiratory CO₂ from 5% to 10%) increased SND (97 ± 6%) and the activity of RVLM neurons (67 ± 4%). Injection of kynurenic acid (Kyn, ionotropic glutamate receptor antagonist) into RVLM or the retrotrapezoid nucleus (RTN) eliminated or reduced PND, respectively, but did not change the effect of CO₂ on SND. Bilateral injection of Kyn or muscimol into the rostral ventral respiratory group (rVRG‐pre‐Bötzinger region, also called CVLM) eliminated PND while increasing the stimulatory effect of CO₂ on SND. Muscimol injection into commissural part of the solitary tract nucleus …