Anesthetic and sedative drugs have been found to diminish the respiratory motor activity of the hypoglossal nerve more than that of the phrenic nerve. This differential depression of motor activity to the upper airway may contribute to the exacerbation of obstructive sleep apnea by sedative drugs. To determine whether morphine has a similar selective action, we recorded phrenic and hypoglossal nerve activities before and after morphine administration in decerebrate, vagotomized cats, paralyzed with gallamine. Morphine diminished the activities of both nerves in most animals, but the responses were highly variable, and no consistent pattern of differential depression was apparent. The variability of the results may reflect the complex nature of opiate actions on the control of breathing.