A series of cyclic conformationally restricted penicillamine containing somatostatin octapeptide analogues have been prepared by standard solid phase synthetic techniques and tested for their ability to inhibit specific [125I]CGP 23,996 (des-Ala1-,Gly2-[desamino-Cys3Tyr11]-dicarba3, 14-somatostatin), [3H]naloxone or [3H]DPDPE ([D-Pen2-D-Pen5]enkephalin) binding in rat brain membrane preparations. We now report structure-activity relationship studies with the synthesis of our most potent and selective mu opioid receptor compound D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, which we refer to as Cys2Tyr3Orn5Pen7-amide. While this octapeptide exhibited high affinity (IC50 = 2.80 nM) for an apparently single population of binding sites (nH = 0.89 +/- 0.1) and exceptional selectivity for mu opioid receptors with an IC50(DPDPE)/IC50 (naloxone) ratio of 4,829, it also displayed very low affinity for somatostatin receptors (IC50 = 22,700 nM). Thus, Cys2Tyr3Orn5Pen7-amide may be the ligand of choice for further characterization of mu opioid receptors and for examining the physiological role of this class of receptors.