Steady-state responses of unanesthetized subjects to hypercapnia or hypoxia provide no evidence that chemoreceptor afferents are differentially distributed upon hypoglossal and bulbospinalphrenic neurons, as suggested by results in anesthetized animals. We hypothesized that dynamic changes in activities of phrenic and hypoglossal nerves might differ following sudden alterations of inspired gases. This hypothesis was based on the observation that episodes of obstructive apnea may follow central apnea. The obstruction might reflect phrenic activity increasing more quickly than that of the hypoglossal upon resumption of ventilation. In decerebrate, vagotomized, paralyzed and ventilated cats, we recorded phrenic and hypoglossal activities before and during abrupt, sustained exposure to hypercapnia, hypoxia and hypoxic hypercapnia, and following sudden withdrawal of these stimuli. For all such manoeuvres, activities of phrenic and hypoglossal nerves increased or decreased in parallel fashion. Our findings cause rejection of the hypothesis of differing dynamic phrenic and hypoglossal responses to chemoreceptor stimuli. The concept that the ventilatory control system is well organized to prevent upper airway obstructions is discussed.