Abstract: Novel compounds which are capable of binding with enhanced specificity to the delta receptor are disclosed. The compounds are a series of cyclic, conformationally constrained analogs of enkephalins which display exceptional delta receptor specificity. The compounds of the present invention are polypeptides of the formula: ##STR1## wherein R.sup.1 and R.sup.2, which may be the same or different, are hydrogen, methyl, or lower alkyl having 1 to 5 carbon atoms;R.sup.3 and R.sup.4, which may be the same or different, are hydrogen, methyl, or lower alkyl having 1 to 5 carbon atoms, provided, however that R.sup.1, R.sup.2, R.sup.3, and R.sup.4 may not all be hydrogen when both n and m are zero;R.sup.5 is hydrogen, L-tyrosine, D-tyrosine, or L-tyrosine or D-tyrosine substituted on the N.sup..alpha. -amino with 1 or 2 lower alkyl or alkenyl groups;R.sup.6 is a substituted or unsubstituted aromatic;R.sup.7 is hydrogen or methyl;R.sup.