Three main types of 5-HT (serotonin) receptor have been recognised. The 5-HT3 receptor is located on neuronal tissues in the peripheral and central nervous systems. Ondansetron is a highly selective and potent antagonist for this receptor type. The severe nausea and vomiting caused by cytotoxic agents and radiotherapy can be reduced by metoclopramide treatment, but extrapyramidal side effects are common due to antagonism of dopamine receptors. Ondansetron has been found to significantly delay the onset of emesis, and reduce the number of retches and vomits in ferrets receiving cisplatin, cyclophosphamide, or radiation, at much lower doses than metoclopramide and without the associated side effects. Experiments to define the site of action of ondansetron suggest that at least part of its antiemetic action is in the area postrema, though a peripheral site of action in the upper gastrointestinal tract is also a possibility.