Abstract: Novel compounds which are capable of binding with enhanced specificity to the mu opioid receptor are disclosed. The compounds are analogs of somatostain and have the formula: ##STR1## wherein X is CONH.sub.2 or CH.sub.2 OH;Y and Z are independently sulfur or CH.sub.2 ;R.sup.1 and R.sup.2, which may be the same or different, are hydrogen, methyl, ethyl, cyclopentamethylene, or a lower alkyl group having five or less carbon atoms;R.sup.3 and R.sup.4, which may be the same or different, are hydrogen, methyl, ethyl, cyclopentamethylene, or a lower alkyl group having five or less carbon atoms, provided, however, that R.sup.1, R.sup.2, R.sup.3, and R.sup.4 may not all be hydrogen;AA.sub.1 is Phe, D-Phe, phenyl-Gly, D-phenyl-Gly, Tyr, D-Tyr, L-1-Naphthylalanine, D-1-Naphthylalanine, or D-Phe(4-Me);AA.sub.2 is Tyr, Phe, Tyr(OMe), Phe(4-Me),Tyr(OEt), or Phe(4-Et); andAA.sub.3 is Lys, Arg, Orn or homo-Arg.

Abstract: Compounds comprising polypeptide derivatives of the formula ##STR1## wherein R.sub.1 is the amino acid sequence of the polypeptide minus the terminal amino acid; R is the side chain group determining the identity of the terminal amino acid; and Z is ##STR2## or --CH.sub.2 X wherein X is Cl, Br, or I; are provided. Precursor peptides are peptide hypothalamic releasing and inhibitory factors, opiate peptides, and fragments thereof. The derivatives act as antagonists or agonists to the precursor peptides and are particularly useful in bioassay procedures.