The occurrence and localization of Met-enkephalin-Arg-Gly-Leu (Met-Enk-Arg-Gly-Leu)-like immunoreactivity in the canine pancreas were studied by immunocytochemistry. For light microscopy, Bouin fixed and paraffin embedded sections were immunostained by the PAP method. For electron microscopy, ultrathin sections of 4% paraformaldehyde-fixed and Araldite embedded materials were immunostained by a protein A-colloidal gold method. The results obtained were as follows: In light microscopy, Met-Enk-Arg-Gly-Leu-like immunoreactivity was localized in nerve fibers of autonomic ganglion, in beaded nerve terminals around blood vessels of the exocrine parenchyma and pancreatic islet, in eighty-five out of one hundred and seventy-four ganglion cell somas of intra- and interlobular ganglia, and in ganglion cell somas and nerve fibers of neuro-insular complexes. In electron microscopy, colloidal gold particles representing Met-Enk-Arg-Gly-Leu-like immunoreactivity were concentrated on secretory type granules of the ganglion cell somas and on large cored synaptic vesicles of the nerve terminals around blood capillaries. All the positive immunoreactions disappeared with preincubation of the anti-Met-Enk-Arg-Gly-Leu serum (dilution, 1:4,000) with Met-Enk-Arg-Gly-Leu (10 micrograms/ml). Immunostaining was not affected by preincubation of the antiserum with Met-enkephalin (10 micrograms/ml), Leu-enkephalin (10 micrograms/ml) and Met-enkephalin-Arg-Phe (20 micrograms/ml). Out of the three precursors for a variety of opioid peptides throughout the body, i.e. preproenkephalin A, preproopiomelanocortin and preproenkephalin B, only preproenkephalin A contains Met-Enk-Arg-Gly-Leu.(ABSTRACT TRUNCATED AT 250 WORDS)