An acid extract of neural tissues of the mollusc, Mytilus edulis, was fractionated by high-pressure chromatography. Peak fractions with retention times of that of Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Phe were subjected to competitive displacement assays in the same neural tissues. The results showed that these fractions have the same binding activities as the authentic neuropeptides. The peptides from these fractions were purified by high-pressure liquid chromatography under isocractic conditions. Sequential amino acid analyses showed these peptides to have the same primary structures as Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Phe.