In order to compare the responses to increasing depths of anesthesia with inhalational anesthetic agents of the phrenic nerve and cranial nerves supplying upper airway muscles, the effects of acute administration of halothane (2.5%) or enflurane (5.0%) on the activities of the hypoglossal nerve (HN), the recurrent laryngeal nerve (RLN), and the phrenic nerve (PN) were investigated in artificially ventilated, vagotomized cats. Following administration of halothane or enflurane, rhythmic inspiratory activities of all three nerves decreased progressively, but the decrease in HN activity was more pronounced than the decreases in HN and RLN activities. During the course of respiratory depression due to increasing depth of anesthesia with halothane or enflurane, rhythmic inspiratory activities of PN and RLN decreased linearly and approximately at the same rate, indicating that the response of RLN more closely resembles that of PN. However, at a relatively deep level of anesthesia, rhythmic inspiratory RLN activity was replaced by strong tonic RLN activity, while PN activity was completely abolished, causing quantitatively and qualitatively different responses of RLN and PN. These peculiar responses of RLN activity to increasing depth of anesthesia were examined in more detail with the use of single- or few-fiber preparations. Increasing depth of anesthesia depressed the inspiratory RLN units while recruiting other units, originally silent, indicating that there are two different groups of fibers in RLN whose response to increasing depth of anesthesia is exactly opposite to one another. The effects of halothane on HN, RLN, and PN were not identical to those of enflurane, since enflurane administration caused characteristic spike-like activities in HN and RLN, which were never observed following halothane, although the overall responses of the activities of all three nerves to increasing depth of anesthesia with halothane were similar to those with enflurane. The dissimilarities observed in all three nerves potentially could affect the balance of activity among upper airway and chest wall muscles and lead to obstruction in the upper airway.