The distribution of mucosal nerve fibres containing vasoactive intestinal peptide (VIP), substance P, somatostatin, neuropeptide Y (NPY), and enkephalinlike immunoreactivity was mapped by conventional immunohistochemical techniques throughout the mucosa of the esophagus, stomach, small and large intestines, and gall bladder. In addition, the distributions of endocrine cells immunoreactive for three peptides localized by these antisera (namely somatostatin, pancreatic polypeptide, and substance P) were recorded. Tissues from guinea pigs, rats, dogs, marmosets, and humans were studied. It was hoped that this information would enable possible target tissues and functional roles for the peptides to be identified. In the mucosa, peptide nerve fibres were found throughout the lamina propria, including some which were close to the epithelium and others associated with small blood vessels. Although there was a general similarity of peptide nerve distribution between regions and species, many small variations were observed. VIP and substance P fibres were the most prevalent nerve type; NPY fibres were also usually quite common. The distribution of somatostatin fibres was extremely variable between regions and species, and enkephalin fibres were usually rare. Endocrine cells of open (flask- or pyramid-shaped) and closed (rounded) types were seen; basal cytoplasmic processes (of variable length) were seen on many cells immunoreactive for somatostatin or pancreatic polypeptide. Epithelial cells immunoreactive for substance P were seen in the dog, marmoset, and human. The distributions and shapes of endocrine cells varied widely between areas and species. These studies provide a basis for the correlation of nerve distribution with pharmacological and physiological studies.