Melatonin (N-acetyl-5-methoxytryptamine), originally discovered in the pineal gland, is now known also to be present in the gastrointestinal tract from the stomach to the colon. It is localized and likely synthesized in the enterochromaffin cells of the mucosal lining. Its functions in the gut generally seem to be protective of the mucosa from erosion and ulcer formation and to possibly influence movement of the gastrointestinal contents through the digestive system. In this brief review, we summarize the work documenting the function of melatonin in influencing bicarbonate secretion in the stomach and its role in preventing and repairing ulcers in the stomach and duodenum. Melatonin's actions in the control of bicarbonate secretion involve the central and peripheral sympathetic nervous systems and the actions are receptor mediated. Conversely, melatonin's actions in reducing ulcer formation also seemingly involve the ability of the indole to directly scavenge toxic oxygen-based reactants, e.g., the hydroxyl radical, and possibly to promote antioxidative enzyme activities. These same processes may be involved in the mechanisms by which melatonin promotes ulcer healing. Additionally, however, melatonin's effects on the healing of ulcers includes actions of blood flow in the margins of the ulcer and also on the sensory nerves. All indications are that melatonin has a variety of beneficial effects in the gastrointestinal tract. It is likely, however, that additional actions of melatonin on the digestive system will be uncovered.