The nucleus accumbens (NAc), a main terminal area of the mesolimbic dopaminergic pathway, receives inputs from orexin neurons. We have shown that intra-accumbal administration of the OX₁ and OX₂ receptor (-R) antagonist MK-4305 or the OX₂-R antagonist EMPA, but not the OX₁ and OX₂-R agonist orexin-A or the OX₂-R agonist orexin-B, induced a dose-related increase in accumbal dopamine (DA) efflux. The increases in DA levels induced by MK-4305 or EMPA treatment were inhibited by orexin-B. These results suggest that the OX₂-R strongly inhibits basal DA efflux in the NAc. However, the role of the OX₁-R in control of accumbal basal DA efflux remains unknown. Therefore, we further examined the effects of the OX₁-R antagonist SB-334867 on basal accumbal extracellular DA levels in freely moving rats using in vivo microdialysis. SB-334867 (20 ng) was applied into NAc though a microinjection needle attached with a dialysis probe and failed to affect basal accumbal DA levels. Taken together, our studies provide neuropharmacological evidence suggesting that accumbal OX₂-R, but not OX₁-R, exerts an inhibitory role in regulating accumbal basal DA efflux in freely moving rats.