Beta-arrestins have been suggested to regulate mu-, delta-, and kappa-opioid receptor-mediated responses. In the present study, we examined the effects of pretreatment with beta-arrestin-2 antibody on tail-flick inhibition induced by opioid receptor agonists in the mouse spinal cord. Intrathecal (i.t.) pretreatment with beta-arrestin-2 antibody potentiated the antinociception induced by i.t.-administered mu-opioid receptor agonists [D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) and endomorphin-1, but not endomorphin-2, the delta-opioid receptor agonist [D-Ala(2)]deltorphin II or the kappa-opioid receptor agonist U50,488H. The present result suggests that beta-arrestin-2 may tonically down-regulate a selected population of mu-opioid receptors activated by endomorphin-1 or DAMGO in the mouse spinal cord.