Abstract: Straight or cyclic pentapeptides with receptor affinity to .mu. or .delta. opioid receptors having a primary sequence backbone of Tyr-X-Phe-Leu-Z, Seq. ID Nos. 1, 2, and 3. X and Z can be covalently coupled to provide a heterocyclic structure according to the following conditions:i) when the pentapeptide is straight X is selected from the group consisting of Ser, Gly, Pro, AMCA and D-Ala and Z is selected from the group consisting of Glu, Gln or amino derivatives of Glu or Gln, Seq. ID No. 1, with the proviso that if X is Ser, then Z is Glu or amino derivatives of Glu; andii) where the pentapeptide is cyclic X is selected from the group consisting of D- or L-2,4-diaminobutyric acid, D- or L-lysine, D- or L-ornithine and D or L-cysteine and Z is selected from the group consisting of Gln or Glu or amino derivatives of Gln or Glu, Seq. ID No. 2, and 3, with the proviso that if X is D-or L-Cys, then Z is Cys, Seq. ID No. 3.