Glycyl-glutamine (Gly-Gln) is an endogenous dipeptide that is synthesized from beta-endorphin post-translationally. Previously, we showed that Gly-Gln prevents acquisition of morphine-conditioned place preference, a behavioral test of morphine reward, but does not interfere with morphine analgesia. In this study, we tested the hypothesis that Gly-Gln inhibits morphine reward by blocking morphine-induced dopamine efflux in the nucleus accumbens (NAc). Extracellular dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were sampled by microdialysis and analyzed by high-performance liquid chromatography with electrochemical detection. Guide cannulas were implanted in the right NAc and left lateral ventricle of male Sprague-Dawley rats stereotaxically. Approximately 24 h later, a microdialysis probe was inserted into the NAc and perfused at 1 microl/min. Gly-Gln (1, 3, 30, or 100 nmol/5 microl) or saline was administered intracerebroventricularly, morphine (2.5 mg/kg) was injected intraperitoneally (i.p.) 2 min later, and extracellular dopamine and DOPAC were sampled at 20-min intervals. Morphine administration increased extracellular dopamine concentrations by approximately 600% within 40 min. Gly-Gln pretreatment inhibited the rise in extracellular dopamine in a dose-related manner; the lowest significantly inhibitory dose was 1 nmol. Gly-Gln also inhibited the morphine-induced rise in extracellular DOPAC concentrations but did not affect extracellular dopamine or DOPAC in control animals. Gly-Gln (100 nmol/5 microl) prevented morphine-induced dopamine efflux in rats treated with morphine chronically (10 mg/kg, i.p. twice daily for 6 days), although it did not affect DOPAC concentrations significantly. These data support the hypothesis that Gly-Gln abolishes the rewarding effect of morphine by inhibiting the ability of morphine to stimulate dopamine release in the NAc.