The cloning and expression of DNA for the three major opioid receptor types (mu, delta, and kappa) present new research opportunities for the characterization of opioid drugs and their interactions with these receptors. Genomic and cDNA clones for opioid receptors exist for several animal species including mouse, rat, guinea pig, and human. These include clones for all three human opioid receptor types. The receptor proteins consist of about 400 amino acids and have the characteristic seven transmembrane domain structure of G-protein-coupled receptors. There is about 60% amino acid identity between opioid receptor types and about 90% identity between a receptor type cloned from different animal species. All opioid receptor types mediate the inhibition of adenylyl cyclase in response to agonist binding. Radioligand binding and functional studies using the cloned receptors tend to support current conclusions on opioid drug receptor selectivity and activity. Investigations of opioid receptor chimeras and single amino acid mutants are providing information on the ligand recognition sites of these receptors and essential support for the development of computational opioid receptor models. A molecular model of the human delta opioid receptor is included in this review.