The distribution of serotonin and serotonin 2B receptor in the small intestines of pigs newborn, 5, 15 and 100 days of age were examined qualitatively and quantitatively by immunohistochemical labeling, microscopic observation and image analysis. The results showed serotonin immunopositive cells distributed diffusely among the epithelial cells of the middle and more basal parts of villi and intestinal glands in all segments of all pigs examined. Serotonin 2B receptor was first localized in the duodenum of 15-day-old pigs, whereas in 100-day-old pigs, serotonin 2B receptor was immunolabeled abundantly in all segments. Serotonin 2B receptor was distributed in the connective tissue of the small intestinal mucosa, lamina propria and in some myenteric neurons. The density of serotonin 2B receptor immunopositive cells in the duodenum of 100-day-old pigs was higher than that of 15-day-old pigs. The density of serotonin 2B receptor immunopositive cells in the duodenum was the highest among the three segments of the 100-day-old pigs. The study indicates that the distribution of serotonin 2B receptor is species different in the pig small intestine and the intensity of serotonin 2B receptor becomes stronger as the small intestine matures.