Melanin-concentrating hormone (MCH) is a cyclic peptide isolated first from salmon brain, then from rat and human hypothalamus. We have recently found expression of MCH messenger RNA and encoded peptides, e.g. MCH and neuropeptide-glutamic acid-isoleucine, within the rat gastrointestinal (GI) tract, but their cellular origin was unclear. Furthermore, similarities in the localization of rat atrial natriuretic factor (ANF) and rat MCH immunoreactivities within intestine suggested functional convergence. In the present study we determined first the presence and distribution of MCH messenger RNA and encoded peptides in the GI tract by combining in situ hybridization and immunohistochemical analysis. Our data revealed numerous MCH-containing cells located in the lamina propria and submucosa at both duodenal and colonic levels. Second, the localisation of MCH- and arginine vasopressin- or ANF-containing cells appears similar at the duodenal and colonic levels, respectively. Colocalization of MCH/neuropeptide-glutamic acid-isoleucine immunoreactivity (-IR) and catecholamine indicated that MCH-expressing cells are probably antigen-presenting cells forming part of the enterochromaffin cell system. Third, we performed reverse phase HPLC coupled to RIA to characterize MCH-like materials in different portions of the rat gut. Crude acidic extracts of rat intestine contained about 2-3 pmol/g tissue of MCH-IR, close to the values found in brain extracts. Reverse phase HPLC of MCH-IR in the GI tract revealed that only 10-30% of the immunoreactivity corresponded to mature MCH, whereas the rat brain contained 94% mature peptide. Finally, we compared the effect of MCH and ANF on water and electrolyte secretions at different levels of the GI tract by using the in situ ligated loop technique. Similar effects were noted for ANF and MCH; both stimulated water, Na, and K fluxes at the proximal colon level and increased Na and K fluxes in the duodenum. However, only ANF increased water and Cl fluxes in the duodenum and decreased bicarbonate secretion in the ileum, whereas MCH increased bicarbonate absorption in the jejunum. The dose required was 10 nmol/100 g.h for MCH, i.e. 10 times more than for the ANF. These studies strongly suggest that MCH produced by antigen-presenting cells of the lamina propria may have an important role, similar to that of ANF at the colonic level, in the physiology of the GI tract.