Abstract 5-Hydroxytryptamine (5-HT) is a mitogen for selected cell types. We have reported that 5-HT is an autocrine growth factor for functioning human pancreatic carcinoid (BON) cells; autocrine growth effect is transmitted by 5-HT 1A but not 5-HT 1C 2 receptors, activation of which decreases cyclic AMP production through a pertussis toxin-sensitive inhibitory GTP-binding protein. In this study, the effect of 5-HT 3 receptor antagonist, ondansetron, on BON was examined. Ondansetron did not affect growth of BON cells and also affected neither stimulation of phosphatidylinositol hydrolysis or inhibition of cyclic AMP production evoked by 5-HT in BON cells. Ondansetron, however, inhibited mobilization of intracellular calcium evoked by 5-HT. Present findings suggest that BON cells possess 5-HT 3 receptors, but their roles in pancreatic carcinoid cells are still unknown.