A series of cyclic, conformationally constrained photolabile peptides related to the enkephalins and to somatostatin were designed and synthesized in an effort to develop highly selective and potent peptides for the delta and mu opioid receptors. The following new peptides were prepared and tested for their delta opioid receptor potency and selectivity in the guinea pig ileum assay, the mouse vas deferens assay, and the rat brain binding assay: H-Tyr-D-Pen-Gly-p-NH2Phe-D-Pen-OH (1, [p-NH2Phe4]DPDPE) and H-Tyr-D-Pen-Gly-p-N3Phe-D-Pen-OH (2, [p-N3Phe4]-DPDPE). The following new peptides were prepared and tested for their mu opioid receptor potency and selectivity in the same assays: H-D-Phe-Cys-p-NH2Phe-D-Trp-Lys-Thr-Pen-Thr-NH2 (3, [p-NH2Phe3]CTP) and D-Phe-Cys-p-N3Phe-D-Trp-Lys-Thr-Pen-Thr-NH2 (4, [p-N3Phe3]CTP). The delta selective photoaffinity peptide 2 displayed both high affinity (IC50 = 9.5 nM) and good selectivity (IC50 mu/IC50 delta = 1053) as an agonist at delta opioid receptors in bioassays, and 2 also displayed moderate affinity (33 nM) and excellent selectivity (IC50 mu/IC50 delta = 110) for rat brain delta opioid receptors. The mu selective photoaffinity peptide 4 displayed very weak affinity (8% contraction at 300 nM) at mu opioid receptors in bioassays, but good affinity (IC50 = 48.6 nM) and excellent selectivity (IC50 delta/IC50 mu = 412) for the rat brain mu opioid receptors. These conformationally constrained cyclic photoaffinity peptides may be useful tools to investigate the pharmacology of delta and mu opioid receptors.