From a consideration of the above evidence, it is possible to hypothesize that the 5-HT3 receptors, which are located both in the gut and in the AP/NTS, may play an important and perhaps pivotal part in the mechanism(s) of action of chemotherapy and radiotherapy to induce emesis in animals and humans and represent the anti-emetic sites of action of ondansetron and related agents (see Fig. 1). The value of ondansetron and the 5-HT3 receptor antagonists has been to greatly improve the treatment of nausea and emesis in the cancer patient and to cause a renaissance in emesis research. The 5-HT3 receptor antagonists have helped to redefine the phases of chemotherapy induced emesis and establish the first clear neurotransmitter links in the emetic reflex. It has also encouraged the analysis of emetic mechanisms that will identify further points for pharmacological intervention that may ultimately provide "broad spectrum" anti-emetic agents. Such compounds would further improve the quality of life and treatment of the cancer patient, leading to increased success in the treatment of malignant tumours.