Contrary to previous reports of morphine's depression of locus coeruleus (LC) unit activity in anesthetized animals, acute administration of morphine (0.5, 2.0 or 4.0 mg/kg, i.p.) did not decrease the unit activity of noradrenergic neurons in the area of the LC of freely moving cats. In fact at the higher doses examined (2.0 and 4.0 mg/kg) morphine significantly increased unit activity. Naloxone (1 mg/kg, i.p.) administration reversed the increase in unit activity produced by morphine. When these same studies were conducted in cats first anesthetized with chloral hydrate, morphine produced a significant decrease in unit activity in a naloxone-reversible manner. These results suggest that previous reports of systemic morphine's depression of LC unit activity may be at least partially attributable to an interaction with anesthesia. Morphine's multiplicity of actions upon the LC is discussed.