Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide involved in neonatal CO2-stimulated respiration. The ventral respiratory column (VRC), which contains the core inspiratory rhythm generator, receives PACAPergic inputs from the retrotrapezoid nucleus (RTN) and expresses PAC1 receptor in neonates. However, in adults, the afferent origins of PACAPergic transmission to the VRC other than the RTN; and PACAP expression changes in response to chronic hypercapnic conditions are unknown.Using cholera toxin subunit B (CTB) mediated retrograde neuronal tracing and in situ hybridization for preproPACAP (ppPACAP) mRNA, we have identified neuronal origins of the PACAPergic afferents to the VRC in adult C57BL/6 mice(n=8). VRC-projecting neurons expressing ppPACAP were found in the motor, somatosensory, insular cortex; medial and lateral preoptic areas, dorsomedial, paraventricular, and lateral hypothalamic area; RTN, area postrema, nucleus of the solitary tract and hypoglossal motor nucleus. Furthermore, we used RT-qPCR to evaluate the effects of 10-day chronic hypercapnic exposures (5 or 8% CO2) compared to control (room air/0% CO2) on ppPACAP expression in the rostral and caudal RTN and the cerebellum. In the rostral RTN, but not the caudal RTN or cerebellum, relative ppPACAP expression was inversely correlated with CO2 exposure. The highest ppPACAP expression was seen in control animals and the lowest in 8% - the relative expression was reduced by 50% compared to control(n=5). These results suggest that PACAPergic neurons in diverse brain regions converge on the VRC of the adult mouse, and that rostral RTN PACAP expression exhibits plasticity in response to chronic CO2 stimulation.