Emesis is a common side effect of chemotherapeutic drugs. Cisplatin, nitrogen mustard and dacarbazine induce increases in urinary 5-hydroxyindoleacetic acid (5-HIAA) in parallel with the development of the period of emesis which is sensitive to 5-HT3 receptor antagonists (‘acute emesis’). It is suggested that these cytotoxics release serotonin from enterochromaffin cells, which then acts on 5-HT3 receptors to trigger the emetic response. Cyclophosphamide, on the other hand, induces a modest emetic response, partly sensitive to 5-HT3 receptor antagonists, but not associated with increases in urinary 5-HIAA. It is suggested that cyclophosphamide-induced emesis is not mediated by the release of serotonin from enterochromaffin cells. Although after high-dose cisplatin most emesis is sensitive to 5-HT3 receptor antagonists, patients often present a milder, although more prolonged form of emesis which is mostly resistant to 5-HT3 receptor antagonists (also known as ‘delayed emesis’)- This form of emesis is not associated with increases in urinary 5-HIAA (not due to serotonin released from the enterochromaffin cells). Treatment with p-chlorophe-nylalanine (a serotonin synthesis inhibitor) inhibited cisplatin-induced emesis and cisplatin-induced increases in urinary 5-HIAA excretion. In summary, these results indicate that in human patients, serotonin plays a fundamental role in chemotherapy-induced emesis. Serotonin released from enterochromaffin cells seems to mediate emesis sensitive to 5-HT3 receptor antagonists induced by cisplatin, dacarbazine and nitrogen mustard. Emesis sensitive to 5-HT3 receptor antagonists associated with cyclophosphamide treatment, is not mediated by the release of serotonin from enterochromaffin cells by the cytotoxic. Therefore, cyclophosphamide could induce serotonin release either from enteric serotonin nerves or from the CNS. Cisplatin-induced emesis resistant to 5-HT3 receptor antagonists (‘delayed emesis’) is not mediated by serotonin released from enterochrom