Two peptides (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-NH₂ and Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH₂) that crossreact with an antiserum raised against Phe-Met-Arg-Phe-NH₂ were purified from bovine brain extract, and the synthetic peptides were found to have the same HPLC retention times as the endogenous Phe-Met-Arg-Phe-NH₂ immunoreactive peptides, thus confirming the assignment of phenylalaninamide to the COOH-terminal positions. Both of the synthetic peptides were found to decrease tail-flick latency in rats, and the octapeptide was more active than the octadecapeptide. The octapeptide was found also to attenuate the prolongation of the tail-flick latency induced by morphine.

The medullae oblongatae and midbrains were dissected and homogenized in10 volumes of 1 M CH₃COOH containing 0.02 M HCl and 0.1% 2-mercaptoethanol.The homogenate was heated (90 ℃) for 10 min and then was centrifuged at 27000 × g for 20 min. The supernatant was decanted and the pellet was extracted with H₂O (5 volumes of original weight of tisue).