SUN-C5174 is a sleective 5-HT2 antagonist (pA2=8.98+/-0.06). SUN C5174 showed a marked inhibitory effect on the platelet aggregation induced by serotonin in combination with collagen and adenosine diphosphate (ADP) in canine or human platelet-rich plasma (IC50=6.5 to 16 nM). SUN-C5174 significantly inhibited the mortality rate in mouse acute pulmonary thromboembolytic death induced by collagen and serotonin at oral doses of 0.3 mg/kg or higher.