Electrochemical methods were used to explore the exocytotic nature of serotonin (5-HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut-derived 5-HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5-HT release in BON cells is 80% compared to previous work in pancreatic beta cells (34%). The fractional release increased from 80% in control BON cells to 87% with 5-HT preincubation and nearly 100% with the combination of 5-HT and the 5HT4 autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5-HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.