Serotonin is widely distributed throughout the gut within both the enteric nerves and enterochromaffin (EC) cells. EC cells are located in the gut mucosa with maximal numbers in the duodenum and rectum where they act as signal transducers, responding to pressure and luminal substances both bacterial and dietary. Activation leads to serotonin release which acts on a range of receptors on mucosal afferent and myenteric interneurones to initiate secretomotor reflexes. These cause nausea and vomiting as well as intestinal secretion, propulsion and if pronounced, diarrhoea. Inflammation in animal models acts via T lymphocytes to increase EC cell numbers and mucosal serotonin (5-HT) content while inflammatory cytokines decrease serotonin transporter (SERT) function. Inflammation due to coeliac disease and following gastrointestinal infection increases mucosal 5-HT availability by a combination of increased EC cells and depressed SERT. Irritable bowel syndrome (IBS) developing after gastrointestinal infection and IBS with diarrhoea is associated with excess 5-HT. The associated diarrhoeal symptoms respond well to 5-HT₃ receptor antagonists. These drugs also inhibit the nausea and vomiting occurring in patients undergoing chemotherapy which cause a marked increase in release of 5-HT as well as other mediators. Other conditions including IBS-C and constipation may have inadequate 5-HT release and benefit from both 5-HT₃ and 5-HT₄ receptor agonists.