The effects induced by orphanin FQ (OFQ) on morphine-induced dopamine (DA), 3,4-dihydroxyphenilacetic acid (DOPAC) and homovanillic acid (HVA) release in the nucleus accumbens were studied in rats by using microdialysis with electrochemical detection. Morphine administered intraperitoneally (i.p., 2, 5 and 10 mg/kg) dose-dependently increased DA and metabolites release in the nucleus accumbens. OFQ intracerebroventricularly (i.c.v.) administered at doses of 2, 5 and 10 nmol did not change DA and metabolites release in the nucleus accumbens. OFQ (10 nmol) administered i.c.v. 15 min before morphine (5 and 10 mg/kg, i.p.) significantly reduced morphine-induced DA and metabolites release in the nucleus accumbens. These effects suggest that OFQ may regulate the stimulant action linked to morphine-induced DA release in the nucleus accumbens.