AbstractThe demonstration of opioid receptors by radioligand binding and the discovery of their endogenous peptide ligands has provided a new class of compounds that can be used for the development of novel opioids. The number of potential receptor targets for such opioids has been expanded by the identification of multiple opioid receptor types The development of highly selective opioid peptides using the principles of conformational restriction permits the analysis of the structure‐activity requirements of each receptor type, and is facilitating the elucidation of the functional properties of the different opioid receptor