Serotonin reactivity in the cardiac, fundic and pyloric regions of the stomach of 20 (ten male and ten female) 2-year old domestic cats (Felis catus L.) was detected by immunohistochemistry. Serotonin immunoreactivity was observed in the epithelium, basement membrane, parietal cells, smooth muscle fibers of the lamina muscularis mucosae and lamina propria, blood vessels in the lamina propria and tunica submucosa, myenteric plexus in the tunica sucmucosa, inner circular and outer longitudinal layers of the tunica muscularis, and blood vessels in the tunica serosa. Serotonin immunoreactive cells were distributed in the apical lining of the fundus but were observed in the basal lining of the surfave epithelium in the cardia and pylorus. Blood vessel reactivity suggests that serotonin can travel by blood to reach its target organs. Serotonin immunoreactivity in the different structures of the stomach suggests its role in regulating gastric secretions and motility.