Serotonin (5-hydroxytryptamine) is a catecholamine with a molecular weight of 176 Daltons that is formed in gastrointestinal enteroendocrine cells, platelets, brain and spinal cord. It is secreted by enterochromaffin cells (EC) in the stomach, intestines and appendix and is believed to play a role in gut motility. Serotonin is the major secretion of carcinoid tumors. It is metabolized with the production of 5-hydroxy-indoleacetic acid.