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 6 and p opioid receptors. The following new peptides were prepared and tested for their 6 opioid receptor potency and selectivity-in the guinea pig ileum assay, the mouse vas deferens assay, and the rat brain binding assay: H ...