The finding that serotonin (5-HT) can modulate dopamine (DA) and norepinephrine (NE) release in the brain has led us to hypothesize that fluoxetine, a selective 5-HT reuptake inhibitor, may influence the ability of bupropion, a preferential DA and NE dual reuptake inhibitor, to modulate extracellular DA and NE concentrations in some brain areas. The present study was designed to evaluate this hypothesis by assessing the effects of fluoxetine on bupropion-induced changes in extracellular monoamine concentrations by means of in vivo microdialysis. Three mesocorticolimbic areas including hypothalamus (Ht), prefrontal cortex (Pfc) and nucleus accumbens (Acb) were selected based on their relevance to depression and antidepressant actions. In the Ht of untreated rats, bupropion dose-dependently (s.c.) increased extracellular DA and NE concentrations either in single injection study or in sequential injection study. Thus, 10 mg/kg of bupropion had no effect on the DA and NE concentrations, while 30 mg/kg of bupropion induced transient but significant increases (about 240% of the baselines), and 100 mg/kg of bupropion induced marked and persistent increases (over 600% of the baselines) in the DA and NE concentrations. In the rats pre-treated with fluoxetine (10 mg/kg, s.c., 90 min interval), the threshold dose of bupropion (10 mg/kg) significantly increased the DA and NE concentrations to more than 350% of the baselines, and 30 mg/kg of bupropion markedly increased the DA and NE concentrations to more than 570% of the baselines in the Ht. The fluoxetine pre-treatment also potentiated the DA increases induced by 10 mg/kg of bupropion in the Pfc (260% for bupropion alone vs 357% for the combination) and in the Acb (224% vs 645%). The bupropion-induced NE increases were potentiated by fluoxetine mainly in the Ht. Bupropion did not significantly affect the extracellular 5-HT concentrations in all the 3 brain areas tested. In summary, the present study demonstrated that bupropion can increase extracellular DA and NE concentrations in several mesocorticolimbic areas, which may have an impact on bupropion's antidepressant actions. Furthermore, fluoxetine can potentiate the bupropion-induced DA and NE increases, which may produce more effective and rapid antidepressant actions.