Bovine pancreatic endocrine cells were investigated by light microscopic immunohistochemistry. Serotonin-immunoreactive cells as well as insulin-, glucagon-, somatostatin-, bovine pancreatic polypeptide (BPP)-immunoreactive cells were detected in the pancreatic islets. Generally, insulin-immunoreactive cells were distributed throughout the islet and the others took peripheral location. Since the distribution and shape of serotonin-immunoreactive cells were very similar to glucagon- and BPP-immunoreactive cells, serial sections were restained by using the elution method. All glucagon- and BPP-immunoreactive cells also showed serotonin immunore activity but glucagon and BPP immunoreactivities were never observed to be colocalized in the same cell. A small number of serotonin-immunoreactive cells were observed that showed serotonin immunoreactivity only.