Sheep .beta.-lipotropin (.beta.-LPH) (sequence 1-91) was selectively cleaved with trypsin after blocking the .epsilon.-amino groups of lysine with citraconic anhydride. The resulting peptides were purified by a combination of cation-exchange chromatography and high-voltage electrophoresis. The purified fragments were then tested for their morphine-like activity in the mouse vas deferens bioassay. The active peptides were 61-91 and 61-80. All other regions of the molecule investigated were not active. Moreover, the peptides 61-91 and 61-80 were about as active as the synthetic methionine-enkephalin, and in turn these were about 100 times more active than .beta.-LPH itself. The inhibition of electrically stimulated mouse vas deferens by these peptides was reversed by naloxone, and suggested a competitive character of interaction. The active core for the morphine-like activity in the mouse vas deferens bioassay was probably the fragment 61-65 of .beta.-LPH.