Acetylcholine (ACh), adenosine 5'-triphosphate (ATP) and sodium cyanide (NaCN) activate petrosal ganglion (PG) neurons in vitro, and evoke ventilatory reflexes in situ, which are abolished after bilateral chemosensory denervation. Because in our previous experiments we superfused the isolated PG with solutions free of CO2 /HCO3¯ buffer, we studied its effects on the PG responses evoked in vitro. PGs from adult cats were superfused at a constant pH, with HEPES-supplemented (5 mM) saline with or without CO2 /HCO3¯ (5% / 26.2 mM) buffer, and carotid (sinus) nerve frequency discharge ( CN) recorded. Increases in CN evoked by ACh, ATP and NaCN in CO2-free saline were significantly reduced (P<0.05, Wilcoxon test) when CO2 / HCO3¯ was present in the superfusion medium. Thus, the presence of CO2 / HCO3¯ buffer appears to reduce PG neurons sensitivity to ACh, ATP and NaCN, an effect that may underlie the lack of ventilatory reflexes after bilateral chemodenervation