They contain about 90% of the body's store of serotonin (5-HT).
In the gastrointestinal tract, 5-HT is important in response to chemical, mechanical or pathological stimuli in the lumen. It activates both secretory and peristaltic reflexes, and activates vagal afferents (via 5-HT3 receptors) that signal to the brain (important in the generation of nausea). Ondansetron is an antagonist of the 5-HT3 receptor and is an effective anti-emetic.
They are stimulated by gastrin, a molecule that is produced at the antrum of the stomach by G cells.
In the Digestive tract they are considered a Histamine.
^Thompson M, Fleming KA, Evans DJ, Fundele R, Surani MA, Wright NA (October 1990). "Gastric endocrine cells share a clonal origin with other gut cell lineages". Development110 (2): 477–81. PMID2133551.