The synthesis and biological testing of analogues of Met‐enkephalin, a recently discovered opioid peptide from mammalian brain, are described. Testing involved determination of affinity constants for an opiate receptor site and of analgesic potency in the tail‐flick test in mouse. The effects on opioid activity of modifying various parts of the enkephalin molecule are discussed. Tyr‐D‐Ala‐Gly‐MePheMet (O)‐ol (FK 33‐824), which was highly active in these tests, was subsequently selected for clinical testing. The use of two complementary models ‐ in vitro binding studies and in vivo test for analgesia ‐ for the assessment of biological activity in the evaluation of analogues is explained. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim