Serotonin (5-HT)-like immunoreactive nerve fibers were investigated in gut tissue obtained from seven Japanese children with Hirschsprung's disease. In the control untreated tissues, 5-HT-like immunoreactive fibers were observed neither in the normoganglionic nor in the aganglionic regions. After pargyline treatment, 5-HT-positive neuropils were consistently detected in association with the myenteric plexus in the normoganglionic segment, while in the aganglionic segment immunoreactive fibers could not be demonstrated through the entire layer of the bowel tissue. The occurrence of 5-HT-like immunoreactive neuropils by pargyline treatment strongly suggests that the infant bowel is innervated with serotoninergic elements. After treatment with 5-hydroxy-L-tryptophan (5-HTP), the immunoreactive neuropils were localized in the myenteric and submucous plexuses of the normoganglionic segment. On the other hand, another type of 5-HT-positive fibers was characteristically demonstrated in aganglionic segments following the drug treatment; moderate numbers of 5-HT-like immunoreactive fibers appeared in the intermuscular zone and within the circular and longitudinal muscle layers. Several 5-HT-positive fibers were present in the hypertrophic nerve bundles seen in a diseased bowel. A possible origin of this type of 5-HT-positive nerve fibers was discussed.