A highly specific monoclonal antibody directed against the C-terminal part of glucagon-like peptide-1 (GLP-1) was raised to immunohistochemically evaluate the distribution of GLP-1 containing cells in the entire gastrointestinal tract including pancreas of rat, pig and man. In the pancreas GLP-1-immunoreactive cells were found variously shaped and predominantly located in the periphery of the islets. Ultrastructurally, GLP-1 was co-localized with glucagon in the alpha-granula of A-cells and was mainly restricted to the electrondense core. In the intestine open type cells reaching the lumen via a slender apical process were stained with the GLP-1 antibody. They occurred in all parts of the crypts but predominantly in the basal portion. The density of GLP-1 immunoreactive cells varied between species in a characteristic order: rat greater than pig greater than man. In pig and human gut a large number of cells occurred in the distal jejunum and ileum. A continuous increase of cell densities was found from the proximal to the distal colon resulting in highest numbers in the rectum. In rats the highest cell density occurred in the ileum. Again, a continuous increase of GLP-1-positive cell numbers was evident from the proximal to the distal portion of small and large bowel. GLP-1 was partly co-localized with PYY. The GLP-1 positive cells appeared electronmicroscopically as L-cells with the typical large granula. This morphological data indicates that GLP-1-releasing cells in the small intestine are appropriately positioned in the distal part to sense and respond to the presence of nutrients that have escaped the absorptive surface of the upper small intestine.