The effects of increasing depths of anesthesia on the activities of the hypoglossal nerve (HN) and the phrenic nerve (PN) were investigated in artificially ventilated, vagotomized cats. An abrupt increase in inspired concentration of halothane from 1% to 4% immediately decreased both HN and PN activities, but HN activity decreased more and disappeared much earlier than did PN activity. Steady-state responses of HN and PN activities to changes in endtidal concentration of halothane showed that halothane depressed both HN and PN activities in a dose-related manner but at different rates, suggesting that respiratory control of the tongue muscles and the diaphragm are in part mediated by different neural pathways. Differential suppression of PN and HN activities also was observed following an acute increase in anesthetic depth with thiopental and diazepam. In contrast, no such differential suppression was observed following ketamine administration. Thus, differential suppression of PN and HN may be associated not only with depth of anesthesia but also with the type of anesthetic used.