The purpose of these studies is to better understand the nature of the reflex interactions that control the discharge patterns of caudal medullary, expiratory (E) bulbospinal neurons. We examined the effect of central chemodrive inputs measured as arterial CO₂ tension (Pa_CO2) during hyperoxia on the excitatory and inhibitory components of the lung inflation responses of these neurons in thiopental sodium-anesthetized, paralyzed dogs. Data from slow ramp inflation and deflation test patterns, which were separated by several control inflation cycles, were used to produce plots of neuronal discharge frequency (F_n) versus transpulmonary pressure (P_t). P_t was used as an index of the activity arising from the slowly adapting pulmonary stretch receptors (PSRs). Changes in inspired CO₂ concentrations were used to produce Pa_CO2 levels that ranged from 20 to 80 mmHg. The data obtained from 41 E neurons were used to derive an empirical model that quantifies the average relationship for F_n versus both P_t and Pa_CO2. This model can be used to predict the time course and magnitude of E neuronal responses to these inputs. These data suggest that the interaction between Pa_CO2 and PSR-mediated excitation and inhibition of F_n is mainly additive, but synergism between Pa_CO2 and excitatory inputs is also present. The implications of these findings are discussed.