The localization and distribution of Met-enkephalin-Arg-Gly-Leu (Met-Enk-Arg-Gly-Leu)-like immunoreactivity in whole mount preparations of different layers of guinea-pig gut (duodenum, jejunum, ileum and proximal colon) were studied by Sternberger's indirect immunocytochemistry. Anti-Met-Enk-Arg-Gly-Leu serum used as the primary antiserum was raised in a rabbit against synthetic Met-Enk-Arg-Gly-Leu coupled to ascaris protein by the glutaraldehyde method. The results obtained were summarized as follows: In the intestinal wall of the guinea-pig, Met-Enk-Arg-Gly-Leu-like immunoreactivity was localized only in nerve elements of the enteric nervous system. In the myenteric plexus, many perikarya showed Met-Enk-Arg-Gly-Leu-like immunoreactivity. Immuno-positive nerve fibers showing varicose appearance were demonstrated not only in the myenteric plexus but also in the longitudinal muscle layer, in the circular muscle layer, in the deep muscular plexus, in the submucous plexus and in the muscularis mucosae. No immuno-positive perikarya were found in the submucous plexus. Met-Enk-Arg-Gly-Leu-like immunoreactivity was not shown in the perivascular plexus and in the mucosa. Compared to those of the duodenum, jejunum or ileum, immuno-positive perikarya in the myenteric plexus of the proximal colon were apparently few in number. The density of a network in the deep muscular plexus formed by immunoreactive fibers was looser in the proximal colon than that in the duodenum, jejunum or ileum. Since Met-Enk-Arg-Gly-Leu-like immunoreactivity-containing nerve elements were densely distributed in the muscular layers of the intestinal wall, it is possible for this nervous system to regulate the contractility of the smooth muscles of the gut.