Evidence has been adduced for the view that the morphiness receptor or receptors in the myenteric plexus of guinea pig ileum are very similar to the brain receptors that mediate the analgesic action of the narcotic alagesics. Such an in vitro model is very suitable for the study of structure-activity relationships because the angonist and antagonist activities of compounds can readily be assessed. One example is the investigation of the effects of changes at the N atom. Another point of interest is the role of substitutions at the C14 atom in morphine and morphinans. Such alterations in structure may have a profound effect on the relative agonist and antagonist activities and may convert a drug with dual agonist and antagonist actions into an antagonist devoid of agonist action.