An investigation of the interaction of stable opioid/ligands and unstable opioid peptides with opioid receptors in guinea pig brain, guinea pig myenteric plexus and mouse vas deferens has been carried out. The initial aim of the study was to further characterise K opioid receptors, using binding assays and isolated tissue bioassays. The second aim was to determine the true affinity and potency of small dynorphin peptides for the K opioid receptor and to determine if metabolism of the peptides to non K opioid receptor-preferring products contributes to their observed in vitro pharmacology.