Abstract: Six rhesus monkeys were used to study the distribution and morphology of 5 - hy-droxytryptamine (5-HT) endocrine cells in the small intestine by using immunohis-tochemical streptavidin/peroxidase (SP) method,and Weibel,s stereological method. The results were as follows:① the density of 5-HT endocrine cells in the small intestine of the rhesus monkeys was the highest in the duodenum and moderate in the jejunum and the lowest in the ileum;② 5-HT endocrine cells were distributed mainly in the glandulate duodenales and less in the villi;③ 5-HT cells were various in shape,mostly oval,shuttle, pyramidal or round ；and ④ basal portion of some endocrine cells had processes,extended to luminal surface of the crypt or intestinal tract with 5-HT positive substance accumulating in their ends. These results suggested that 5-HT endocrine cells have paracrine and exocrine functions.