A series of cyclic, conformationally constrained peptides related to somatostatin were designed and synthesized in an effort to develop highly selective and potent peptides for the mu opioid receptor. The following new peptides were prepared and tested for their mu opioid receptor potency and selectively in rat brain binding assays: D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (2, CTOP); D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (3, CTAP); D-Phe-Cys-Tyr-D-Trp-Nle-Thr-Pen-Thr-NH2 (4); D-Phe-Cys-Tyr-D-Trp-Lys-Val-Pen-Thr-NH2 (5); D-Phe-Cys-Tyr-D-Trp-Lys-Gly-Pen-Thr-NH2 (6); D-Phe-Cys-Tyr-Trp-Lys-Thr-Pen-Thr-NH2 (7); D-Tyr-Cys-Tyr-D-Trp-Lys-Thr-Cys-Thr-OH (8); D-PhGly-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (9); and D-PhGly-Pen-Phe-D-Trp-Lys-Thr-Cys-Thr-OH (10). The most selective peptide, 2 (CTOP), displayed both high affinity (IC50 = 3.5 nM) and exceptional selectivity (IC50 delta/IC50 mu = 4,000) for mu opioid receptors. Furthermore, 2 exhibited very low affinity for somatostatin receptors in the rat brain (IC50 greater than 24,000 nM), with an IC50 somatostatin/IC50 mu receptor selectivity of 8,750. These conformationally constrained cyclic peptides should provide new insight into the structural and conformational requirements for the mu opioid receptor and the physiological role of this receptor.