The central action of peptides to influence GI motility in experimental animals is summarized in Table 1. TRH stimulates gastric, intestinal, and colonic contractility in rats and in several experimental species. A number of peptides including calcitonin, CGRP, neurotensin, NPY, and mu opioid peptides act centrally to induce a fasted MMC pattern of intestinal motility in fed animals while GRF and substance P shorten its duration. The dorsal vagal complex is site of action for TRH-, bombesin-, and somatostatin-induced stimulation of gastric contractility, and for CCK-, oxytocin- and substance P-induced decrease in gastric contractions or intraluminal pressure. The mechanisms through which TRH, bombesin, calcitonin, neurotensin, CCK, and oxytocin alter GI motility are vagally mediated. An involvement of central peptidergic neurons in the regulation of gut motility has recently been demonstrated in Aplysia, indicating that such regulatory mechanisms are important in the phylogenesis. Alterations of the pattern of GI motor activity are associated with functional changes in transit. TRH is so far the only centrally acting peptide stimulating simultaneously gastric, intestinal, and colonic transit in various animals species. Opioid peptides acting on mu receptor subtypes in the brain exert the opposite effect and inhibit concomitantly gastric, intestinal, and colonic transit. Bombesin and CRF were found to act centrally to inhibit gastric and intestinal transit and to stimulate colonic transit in the rat. The antitransit effect of calcitonin and CGRP is limited to the stomach and small intestine. The delay in GI transit is associated with reduced GI contractility for most of the peptides except central bombesin that increases GI motility. Nothing is known about brain sites through which these peptides act to alter gastric emptying and colonic transit. Regarding brain sites influencing intestinal transit, TRH-induced stimulation of intestinal transit in the rat is localized in the lateral and medial hypothalamus and medial septum. The periaqueductal gray matter is a responsive site for mu receptor agonist- and neurotensin-induced inhibition of intestinal transit. The neural pathways from the brain to the gut whereby these peptides express their stimulatory or inhibitory effects on GI transit is vagal dependent with the exception of calcitonin. It is not known whether the vagally mediated inhibition of GI transit by these peptides results from a decrease activity of vagal preganglionic fibers synapsing with excitatory myenteric neurons or an activation of vagal preganglionic neurons synapsing with inhibitory myenteric neurons. The lack of specific antagonists for these peptides has hampered the assessment of their physiological role.(ABSTRACT TRUNCATED AT 400 WORDS)