Inactivation of dynorphin-(1-8) in three in vitro isolated preparations, guinea-pig ileum, mouse vas deferens and rabbit vas deferens, was estimated by employing the relatively specific inhibitors of enkephalin-hydrolyzing enzymes. All three enzyme inhibitors, amastatin, captopril and phosphoramidon, significantly enhanced the inhibitory potency of dynorphin-(1-8) in the three isolated preparations. The magnitude of the enhancement of the dynorphin potency by captopril was significantly higher than that by either amastatin or phosphoramidon in guinea-pig ileum; that by amastatin was significantly higher than that by either captopril or phosphoramidon in rabbit vas deferens; and that by amastatin was similar to that by captopril, but significantly higher than that by phosphoramidon in mouse vas deferens. The Ke values of three antagonists, naloxone, Mr 2266 and ICI 154129, against dynorphin-(1-8) in the presence of the three peptidase inhibitors indicated that dynorphin-(1-8) acted on kappa receptors in guinea-pig ileum and on both kappa and delta receptors in mouse vas deferens. Since amastatin, captopril and phosphoramidon produced the naloxone-reversible inhibition of contractions of guinea-pig ileum in the presence of dynorphin-(1-8), all three dynorphin-inactivating enzymes were indicated to be located very close to kappa receptors.