Met-enkephalin-like immunoreactivity-containing nerve elements in the canine pancreas was investigated by immunocytochemistry. Anti-Met-enkephalin (Met-Enk) serum used as the primary anti-serum was raised against synthetic Met-Enk conjugated with bovine serum albumin by a glutaraldehyde method. For light microscopic immunocytochemistry, Bouin-fixed and paraffin-embedded sections not exceeding 20 micrometers in thickness were stained by the PAP method. For electron microscopy, the fixative used was 4% paraformaldehyde plus 1% glutaraldehyde in a 0.067 M phosphate buffer. Ultra-thin sections of Araldite-embedded materials were stained by a protein A-colloidal gold method. The main localization of Met-Enk-like immunoreactivity-containing nerve fibers and ganglion cell somas was as follows: a) on the surface of ganglion cell somas, b) among nerve fiber bundles in the connective tissue septum, c) around blood vessels in the exocrine parenchyma, and d) in the islet of Langerhans. Electron microscopic immunocytochemistry showed that colloidal gold particles representing a Met-Enk-like substance were concentrated on secretory type granules of the ganglion cell somas and nerve terminals around the blood capillaries. Met-Enk-like immunoreactivity was eliminated by preincubation of the primary anti-serum with Met-Enk (nerve fibers: 0.1 X 2(-3) g/l, ganglion cells: 0.1 X 2(-2) g/l of the diluted antiserum, 1:2000) before application to tissue sections. The functional significance of Met-Enk-like immunoreactivity-containing nerve elements in both the endocrine and exocrine pancreas was discussed.