The inhibitory effects of opioid peptides such as [Met5]-enkephalin and [Met5]-enkephalin-Arg6 on the electrically-evoked contractions of guinea-pig ileum, mouse vas deferens and rat vas deferens were enhanced by aminopeptidase inhibitors such as amastatin and bestatin, a peptidyl dipeptidase A inhibitor like captopril, and endopeptidase-24.11 inhibitors such as phosphoramidon and thiorphan. The magnitude of the enhancement by each peptidase inhibitor depended on both the preparation and opioid peptide employed. Additionally, enkephalin had been shown to be almost exclusively hydrolyzed at least in the ileal and striatal guinea pig membrane preparation by 3 kinds of enzymes, amastatin-sensitive aminopeptidase(s), captopril-sensitive peptidyl dipeptidase A and phosphoramidon-sensitive endopeptidase-24.11, which were indicated to be located very close to opioid receptors.