Neurokinin 1 receptors (NK1R) in the cNTS are involved in the hypoxic ventilatory response and also associated with areas of central CO2-sensitivity such as cNTS. We tested the hypothesis that chronic stimulation of either chemoreflex will increase NK1R expression in the cNTS. We acclimatized adult male rats to 7 days of hypobaric hypoxia (HX, PIO2 = 70 Torr, n = 6) or normobaric hypercapnia (HC, FICO2 = 5%, n = 6) and measured ventilatory responses with whole body plethysmography. In room air before acclimatization, V = 374 ± 44 ml/min*kg. After 7 days of HX, V = 1171 + 135 ml/min*kg and V = 642 ± 77 ml/min*kg after 7 days of HC. V in HC showed a tendency to decrease from 1045 ± 146 ml/min*kg on day 1. NK1R immunoreactivity (NK1R-ir) on 30 micron frozen transverse sections of cNTS was increased by 7 days of HX but not HC. NK1R-ir occurred mainly on fibers instead of cell bodies. Total NK1R protein measured in micropunches from the cNTS with Western blotting increased 197% in chronic HX (n=6) and similar experiments are being done for chronic HC. Results to date indicate that tonic increases in arterial chemoreceptor afferent input or hypoxia up-regulate NK1R-ir in the cNTS but tonic central chemoreceptor stimulation does not.