Abstract 1. 1. Interactions of codeine-epoxide, a new metabolite of codeine, with multiple opiate receptors were studied using radioligand binding assay. 2. 2. The ability of codeine-epoxide to displace [ 3H]dihydromorphine binding was the most effective among three labeled ligands, that is, [ 3H]dihydromorphine, [ 3H]- d-Ala 2- d-Leu 5-enkephalin and [ 3H]ethylketocyclazocine, suggesting that codeine-epoxide had a selectively high affinity to μ-receptors, despite the fact the affinities of codeine-epoxide to opiate receptors were slightly less potent than its parent compound, codeine. 3. 3. Since “sodium ratio” and “GTP ratio” on codeine-epoxide binding lay between those of codeine and naloxone, the interaction of codeine-epoxide with opiate receptors may be slightly different from that of codeine.