A comparison was made between the affinities of a wide range of opiate agonists, mixed agonist-antagonists and antagonists for opiate receptor binding sites in the guniea-pig intestine longitudinal muscle and myenteric plexus preparation, and their pharmacological potency in influencing the electrically induced contraction of this in vitro functional system. The relative affinities of drugs and the degree of stereospecificity for intestinal binding sites are closely similar to these properties in the brain. Receptor binding correlates extremely well with pharmacological potency, both for agonists and antagonists, indicating that binding involves pharmacologically relevant opiate receptors. Pharmacological activity correlates best with receptor binding assayed in the presence of sodium.