FK1052, (+) -8,9-dihydro-10-methyl-7-「(5-methyl-4-imidazolyl) methyl」pyrido「1,2-a」indol-6(7H)-one hydrochloride, showed dose-dependent antagonism against bradycardia induced by 2-methyl-5-HT in anesthetized rats. PK1052 had a lower ED_50 value (0.3 μg/kg, i.v. and 4.8 μg/kg, i.d.) than either ondansetron (5.2 μg/kg, i.v. and 170 μg/kg, i.d.) or granisetron (0.7 μg/kg, i.v. and 66 μg/kg, i.d.). PK1052 was a potent and effective agent against cisplatin (CDDP)-induced emesis in dogs. PK1052, given p.o. or i.v. before and after CDDP, was highly protective against CDDP-evoked emesis (ED_50 = 2.7 μg/kg, p.o. and 1.1 μg/kg, i.v.), when compared with ondansetron (ED_50 =31 μg/kg, p.o. and 13 μg/kg, i.v.) and granisetron (ED_50 =14 μg/kg, p.o. and 2.6 μg/kg, i.v.). When FK1052 (100 μg/kg, i.v.) was given 4 hr before CDDP, emesis was significantly prevented. Neither ondansetron nor granisetron, at 100 μg/kg, i.v., administered 30 min before CDDP prevented vomiting. Furthermore, FK1052, given after the onset of CDDP-induced emesis, reduced the number of subsequent emetic episodes (ED_50 = 3.2 μg/kg, i.v.). These results suggested that FK1052 is a potent long-acting and orally effective 5-HT3 receptor antagonist .