The occurrence and localization of Met-enkephalin-Arg-Gly-Leu (Met-Enk-Arg-Gly-Leu)-like immunoreactivity in the lumbar paravertebral ganglion and the superior mesenteric ganglion in the rat were investigated by immunocytochemistry. Colchicine was employed as an axonal flow blocker to accumulate Met-Enk-Arg-Gly-Leu in the nerve cell bodies. The Met-Enk-Arg-Gly-Leu serum (R 0171) was produced by immunizing a mixed breed female rabbit with synthetic Met-Enk-Arg-Gly-Leu conjugated to ascaris protein by the glutaraldehyde method. Sections, 7 micron in thickness, of Bouin-fixed and paraffin-embedded tissues were immunostained by the peroxidase-antiperoxidase (PAP) method. Results obtained were summarized as follows: Colchicine-untreated rats possessed no strongly-immunoreactive nerve cell bodies for Met-Enk-Arg-Gly-Leu in the lumbar paravertebral ganglion, though a few of them were weakly immunopositive. Met-Enk-Arg-Gly-Leu-like immunoreactivity showing nerve fibers were occasionally seen in nerve fiber bundles of the lumbar paravertebral ganglion. In the superior mesenteric ganglion, a dense network of positively immunostained nerve fibers with a beaded appearance were seen around nerve cell bodies which were immunonegative for Met-Enk-Arg-Gly-Leu. In colchicine-treated rats, 55 percent of nerve cell bodies of the lumbar paravertebral ganglion showed various degrees of positive immunoreactivities for Met-Enk-Arg-Gly-Leu. Many strongly immunostained nerve terminals were seen in the lumbar paravertebral ganglion of colchicine-treated rats. In the superior mesenteric ganglion, the immunoreactivity for Met-Enk-Arg-Gly-Leu in the beaded nerve fibers around nerve cell bodies was intensified by the colchicine treatment, whereas the nerve cell bodies themselves remained immunonegative for Met-Enk-Arg-Gly-Leu.