The effect of acute morphine injection (4 or 40 mg/kg, i.p.) on serotonergic (5-HT) and catecholaminergic (CA) neurons in several areas of the rat brain was studied using the Falck-Hillarp histofluorescence method. Changes in fluorescence intensity (i.e. the changes in amine content) were only found in a few discrete regions. Biphasic changes occurred; an initial decrease (after 40 min) was followed by an increase (after 90 min) in amine content of 5-HT nerve cell bodies and CA terminals in the dorsal raphe nucleus and in the region of group B3. Increases were also observed after 90 min in 5-HT nerve cell bodies in the median raphe nucleus and terminals in the nucleus linearis caudalis, and in CA nerve cell bodies in the pars compacta of the substantia nigra and the ventral tagmental area. No significant changes in fluorescence were observed after two weeks chronic morphine treatment. The changes in fluorescence after acute morphine treatment occurred in structures involved in analgesic function. The time-course of these changes suggests that morphine increases both the release and synthesis of 5-HT and CA in these structures.