In the latter part of the 20th century, significant advances have been made in the understanding of the emetic reflex. As a result, there have been major improvements in the treatment of vomiting, particularly that associated with chemo- and radiotherapeutic treatments for neoplastic disease. The 5-HT3 receptor antagonists (ondansetron and granisetron) have been demonstrated to be of benefit in treating the profound emesis observed during cancer treatment. This observation, together with results from pharmacologic and physiologic investigations in both animals and humans, have identified 5-hydroxy-tryptamine (5-HT or serotonin) to be of fundamental importance in the pathogenesis of emesis. 5-HT appears to be released by radiation and chemotherapeutic agents from enterochromaffin cells within the wall of the intestine, and possibly from neurons within the brainstem. Stimulation of 5-HT3 receptors, located centrally in the dorsal medulla of the brainstem and peripherally on vagal afferent terminals in the gastrointestinal tract, appears to play a pivotal role in eliciting emesis. The interaction of 5-HT with non-5-HT3 receptors, particularly 5-HT1A and 5-HT4 receptors, may be important in the emetic reflex. The development of agents that interact with these receptors may offer alternative approaches to the treatment of nausea and vomiting.