Central chemoreception is the mechanism by which the brain senses changes in CO2/H+ to regulate breathing. The cellular identity of chemoreceptors has not been definitively established and the molecular substrate(s) for pH-sensitivity in chemosensory neurons remain largely unknown. Here, we identify phenotypically distinct neurons within the retrotrapezoid nucleus (RTN) as chemosensors and show that they express a pH-sensitive background K+ current different from TASK channels which accounts for their chemosensitivity. Further, we show that pH sensitivity of serotonergic raphé neurons is essentially eliminated in TASK channel knockout mice even as ventilatory responses to CO2 are fully retained, effectively dissociating chemosensitivity of raphé neurons from respiratory chemosensitivity. We recently identified a population of CO2-sensitive neurons in the RTN that appear to function as specialized chemoreceptors. These neurons are intrinsically sensitive to CO2 in vivo, are glutamatergic and send excitatory projections to the respiratory rhythm generator (1). In addition, CO2-sensitive RTN neurons express Phox2b (2), a transcription factor mutated in a congenital central hypoventilation syndrome characterized by a specific deficit in chemosensitivity (3). In a medullary brain slice preparation we identified a group of intrinsically pH-sensitive neurons in the RTN that share morphological features with those recorded in vivo (1). In addition, we found that these RTN neurons express a weakly-rectifying pH-sensitive K+ current with properties reminiscent of the TASK channel subfamily of K2P background K+ channels. The purpose of the current work is twofold: to confirm that pH-sensitive RTN neurons identified in vitro constitute the same group of chemoreceptors identified in vivo; and to test if TASK channels contribute to the pH sensitivity of these RTN chemoreceptors and to respiratory chemosensitivity. We took advantage of our observation that Phox2b could serve as a marker for RTN chemosensitive neurons, as described in vivo. Chemosensitive RTN neurons were identified in vitro by making loose patch current clamp recordings of Vm; these cells were spontaneously active at pH 7.3, inhibited by pH 7.5 and activated by pH 6.9. There was no difference in pH sensitivity of RTN neurons from age-matched rat and mouse pups (P7-12). Following functional characterization, we harvested the cytoplasmic contents of pH-sensitive neurons and performed single cell RT-PCR using outside and nested primers for Phox2b (and GAPDH as a control). Of neurons for which control RNA was detected (N=8), we found that most also expressed Phox2b (N=6). Along with other common characteristics (anatomical localization, morphological features, pH sensitivity), the shared expression of Phox2b strongly suggests that pH-sensitive cells recorded in vivo and in vitro represent the same population of RTN chemosensory neurons. We used pharmacological and genetic approaches to test if TASK channels contribute to the pH-sensitive current in RTN chemoreceptors. The inhalation anesthetic halothane, which typically activates TASK channels in vitro, inhibited a K+ conductance in pH-sensitive RTN neurons in rats and increased their firing rate. In addition, halothane had no effect on the pH-sensitive K+ current expressed by rat RTN neurons. Further, acidification activated RTN neurons normally in mice with deletions of TASK-1, TASK-3 or both subunits and the pH-sensitive background K+ current of these cells was the same in control and knockout mice. Together, these data indicate that TASK channels contribute little to the pH-sensitive resting K+ current of RTN chemoreceptors from rat or mouse. By contrast, bath acidification no longer activated serotonergic raphé neurons in slices from TASK channel knockout mice and the pH-sensitive background K+ current previously attributed to TASK channels in these neurons was eliminated. Although raphé neurons were no longer activated in vitro by acidification in TASK knockout mice, the respiratory response of these animals to CO2 was normal in the awake state, as judged by whole animal plethysmography. In conclusion, TASK channel underlie the pH sensitivity of serotonergic raphé neurons in slices but these channels are not necessary for the pH sensitivity of RTN neurons in vitro nor for central respiratory chemosensitivity in vivo. These results supports the possibility that RTN neurons are chemoreceptors and they suggest that the pH sensitivity of TASK channels and serotoninergic neurons contributes little to respiratory chemosensitivity.