Development of the serotonin type 3 (5-HT3) receptor antagonists has facilitated a new understanding of the physiology of emesis. Three new agents, ondansetron, granisetron and tropisetron, are currently available in several countries and this review will focus on the characteristics of ondansetron and granisetron, so that a perspective of the data for tropisetron can be presented by other authors. Preclinical data indicate some possible differences among these new compounds. They differ in pharmacokinetic terms. Efficacy and a good safety profile for acute emesis control have been demonstrated for both agents, although there may be some debate as to the best schedule of ondansetron. Delayed emesis does not seem to be adequately controlled by these drugs, and they may also lose some efficacy during multiple-day chemotherapy or after several cycles of chemotherapy. While the addition of corticosteroids seems to increase the response rate to ondansetron, no such data are yet available for granisetron. Finally, considerations pertinent to the cost of these new agents are discussed.