Abstract: The distribution of 5-hydroxytryptamine (5-HT) immunostaining cells in digestive tracts of vertebrates containing Ophiocephalus argus,Bufo bufo gargarizans,Clemmys mutica,Melopsittacus undulatus and Mus musculus albula were studied by peroxidase-antiperoxidase (PAP) method.The immunostaining cells were localized in the gastrointestinal tracts of above animal (except the gaster of Melopsittacus undulatus,the intestine and the cardia of gaster of Ophiocephalus argus).It first were localized in the esophagus of Clemmys mutica and B.b.gargarizans.Generally,the density of 5-HT immunostaining cells in the gastrointestinal tracts were the highest in the gasters and moderate in the duodenum and the large intestine,and the lowest in small intestine.The 5-HT immunostaining cells were visualized to distribute between the epithelum cells of the mucosa or gland.The positive cells usually had one or more processes which contained 5-HT immunostaining substance.Some processes were reached into lumen surface of the gland or intestine.Some were extended between other epithelum cells or even reached into lamina propria through the basal membrane.These results indicate that the 5-HT immunostaining cells in digestive tracts could release 5-HT by both endocrine and exocrine ways.