Naloxone (an opiate antagonist) inhibited the black-background adaptation of bullfrog tadpoles Rana catesbeiana. The melanophore index (MI) of naloxone-injected tadpoles was significantly lower than that of controls 1hr after changing the background from white to black. Naloxone was not effective in lightening the skin color in the hypophysectomized tadpoles bearing a pituitary graft. Their skin color remained dark. Tadpoles injected with pimozide (a dopamine antagonist) or α-methyl-p-tyrosine (α-MPT, catecholamine synthesis inhibitor) simultaneously with naloxone showed significantly higher MI than that of the animals treated with naloxone alone. The present results seem to indicate that opioid peptides exert an influence on MSH release in background response of bullfrog tadpoles by modulating MSH release inhibiting activity of the hypothalamic dopamine neurons.