We have studied the response of the pneumostome to CO2, O2 and combined CO2 and O2 in intact snails. We found that pneumostomal opening increases in response to both hypercapnia and mild hypoxia. We determined which neural structures were essential for the pneumostomal response to CO2 by eliminating parts of the nervous system: the subesophageal ganglia and an intact anal nerve were necessary and sufficient elements for the CO2 response. Within the subesophageal ganglia, we identified a discrete region on the medial margin of the visceral ganglion that was capable of increasing pneumostomal area when focally stimulated with 6% CO2. Ion substitution experiments indicated that pneumostomal responses to hypercapnia were not mediated by the pneumostomal motor neurons themselves, but rather by interneurons connected polysynaptically to the motor neurons controlling pneumostomal function. In conclusion, intact H. aspersa have a ventilatory response to CO2, and this response is mediated by CO2 sensitive cells located in a small area of the central nervous system.