In an effort to determine the effect of side chain conformational restriction on opioid receptor selectivity, the cyclic phenylalanine analogues 2-aminoindan-2-carboxylic acid (Aic), 2- aminotetralin-2-carboxylic acid (Atc), and tetra-hydroisoquinoline-3-carboxylic acid (Tic) were substituted for Phe in the potent cyclic opioid peptide analogue H-Tyr-~-Orn-Phe-Glu- NH~, which lacks significant opioid receptor selectivity. Compounds were tested in p-and ...