Antiemetic effects of serotonin 5-HT3 receptor antagonists (ICS205-930, zacopride, BRL43694, GR38032F) were investigated in Suncus murinus. Veratrine, nicotine, copper sulfate, cisplatin, cyclophosphamide and motion sickness were used as emetic stimuli. Serotonin 5-HT3 receptor antagonists did not inhibit emetic responses to veratrine, nicotine, copper sulfate and motion sickness. However, cisplatin- and cyclophosphamide-induced emesis was strongly blocked by them. Both subcutaneous and intravenous injections of 5-HT3 antagonists were effective. Serotonin 5-HT1 and 5-HT2 receptor antagonists were less effective. These results clearly indicate that a 5-HT3 receptor-mediated mechanism(s) is involved in the emesis caused by cancer chemotherapeutic agents and that 5-HT3 receptor antagonists are very effective as prophylactic drugs.