The effects of duloxetine (LY248686), a new inhibitor of serotonin (5-hydroxytryptamine; 5-HT) and norepinephrine (NE) uptake on extracellular levels of NE, 5-HT and dopamine (DA), were studied in the rat frontal cortex and nucleus accumbens, using in vivo microdialysis. The oral administration of duloxetine (3.125-12.5 mg/kg) produced a dose-dependent increase in the output of both NE and 5-HT from the frontal cortex, this increase being maintained throughout the 4-hr observation period. Chronic administration of duloxetine (6.25 mg/kg, p.o.) for 14 days failed to alter basal NE and 5-HT levels in the frontal cortex, but augmented the duloxetine-induced increase in output of NE and 5-HT. Amitriptyline and maprotiline, administered p.o. at doses of 6.25 to 25 mg/kg, increased NE output, but the effect was weaker than that of duloxetine, and neither amitriptyline nor maprotiline changed 5-HT output from the frontal cortex. Duloxetine, amitriptyline, and maprotiline brought about increases in DA levels in the rat frontal cortex, the increase in DA levels induced by duloxetine being approximately two or three times more potent than that induced by amitriptyline and maprotiline. In the nucleus accumbens, duloxetine also produced a dose-dependent increase in DA output more potent than that produced by amitriptyline and maprotiline. These results show that duloxetine causes a potent and sustained increase in the output of both NE and 5-HT in the rat frontal cortex, related to its inhibition of NE and 5-HT uptake; these results also show that duloxetine increases DA output in the frontal cortex.