The sixth lumbar and first sacral spinal cord segments in the rat contain parasympathetic preganglionic neurons which innervate the pelvic viscera. There have been few studies, however, which have specifically considered the distribution of putative peptide neurotransmitters in these cord segments. The present paper describes and compares the immunohistochemical distribution of dynorphin (1-8)-,enkephalin-, somatostatin-, cholecystokinin octapeptide-, avian pancreatic polypeptide-, FMRF-NH2-,neurotensin-, and vasoactive intestinal polypeptide-like immunoreactivities in the dorsal gray commissure and sacral parasympathetic nucleus of the sixth lumbar and first sacral spinal cord segments in colchicine-treated rats. Antisera against all of the peptides, except avian pancreatic polypeptide, stained cells in the sacral parasympathetic nucleus. Dynorphin (1-8-), enkephalin-, and substance P-like immunoreactive cells were present in significantly greater numbers than somatostatin-, neurotensin-, cholecystokinin-, FMRF-NH2-,and vasoactive intestinal polypeptide-like immunoreactive cells. All of the antisera also stained fibers in the sacral parasympathetic nucleus in varying densities, and a fiber bundle which extended between the dorsal gray commissure and the sacral parasympathetic nucleus. Antisera against substance P and cholecystokinin stained a bundle of fibers that extended between the dorsal horn and the sacral parasympathetic nucleus. Antisera against somatostatin, cholecystokinin octapeptide, substance P and FMRF-NH2 stained an additional fiber bundle which extended between the lateral edge of the dorsal horn and the dorsal gray commissure. All the remaining antisera, except neurotensin, also stained fibers that extended between the sacral parasympathetic nucleus and the dorsal gray commissure, but in a sparser distribution. Immunoreactive cells were localized to the dorsal gray commissure in sections stained with each of the antisera. Dynorphin (1-8) and enkephalin antisera stained the greatest number of cells, followed by FMRF-NH2, neurotensin, somatostatin and avian pancreatic polypeptide. The smallest number of immunoreactive cells was present in substance P, cholecystokinin and vasoactive intestinal polypeptide immunostained sections. A significant difference was noted between the number of dynorphin, enkephalin, somatostatin, cholecystokinin, avian pancreatic polypeptide, FMRF-NH2, neurotensin and vasoactive intestinal polypeptide immunoreactive cells in the sacral parasympathetic nucleus and dorsal gray commissure. Plexuses of immunostained fibers were also present in varying densities in the dorsal and ventral portions of the dorsal gray commissure of sections stained with each of the antisera. All of the peptides except vasoactive intestinal polypeptide were also localized to a longitudinal bundle of fibers just dorsal to the central canal. The neurotensin-like immunoreactive fiber bundle was less dense than fibers stained with the other antisera and only present in the first sacral segment. A longitudinal fiber bundle ventral to the central canal was identified in sections immunostained with all of the antisera except neurotensin. Immunostained fibers located along the lateral edge of the dorsal funiculus and extending between the dorsal horn and dorsal gray commissure were seen in sections stained with all of the antisera. It is not clear what function is mediated by the putative peptide neurotransmitters located in these immunostained cells and fibers, however, their close association with autonomic structures in these levels suggests that they may be intimately involved in the parasympathetic regulation of pelvic viscera.