SUMMARY The regional distribution and relative frequency of argyrophil cells, and of cells immunoreactive for 5-hydroxytryptamine (5-ht), substance P (sp), somatostatin, glicentin, glucagon, bovine pancreatic polypeptide (bpp), gastrin, leucine-enkephalin, gastric inhibitory polypeptide (gip), cholecystokinin, secretin, motilin, and neurotensin were studied in 9 segments from the gastrointestinal tract of cows (> 1 year old) and calves (< 3 months old). Argyrophil cells, 5-ht-immunoreactive cells, and somatostatin-immunoreactive cells were distributed throughout the gastrointestinal tract, whereas the other immunoreactive cells were more restricted in distribution. Most endocrine cells were more numerous in the calf than in the cow. This feature was most conspicuous in the abomasum. In the abomasum, argyrophil cells in the cow and calf and 5-ht-immunoreactive cells in the calf were found predominantly in the fundic region, whereas somatostatin-immunoreactive cells and gastrin-immunoreactive cells in the cow and calf and 5-ht-immunoreactive cells in the cow were most numerous in the pyloric region. Substance P-, glucagon-, bpp-, and leucine-enkephalin-immunoreactive cells were rarely detected. In the small intestine, argyrophil cells, 5-ht-, sp-, somatostatin-, gastrin-, gip-, cholecystokinin-, secretin-, and motilin-immunoreactive cells were most numerous in the duodenum. Neurotensin-, glicentin-, glucagon-, and bpp-immunoreactive cells were detected with the highest frequency in the ileum. In the large intestine, argyrophil cells and 5-ht-, glicentin-, bpp-, somatostatin-, glucagon-, and sp-immunoreactive cells occurred with the highest frequency in the rectum.