We examined the antithrombotic and thrombolytic effects of the G(q/11) inhibitor YM-254890 in an electrically-induced carotid artery thrombosis model in rats. YM-254890 dose-dependently inhibited ex vivo ADP-induced platelet aggregation after i.v. bolus injection. In the thrombosis study, YM-254890 dosedependently prolonged time to occlusion at doses of 3 and 10 g/kg i.v. and decreased occlusion rate at 10 g/kg i.v. In the thrombolysis study, YM-254890 at 30 micro g/kg i.v. shortened the time to reperfusion and prevented reocclusion after thrombolysis with a modified tissue-type plasminogen activator. YM-254890, at 10 micro g/kg and more, significantly improved carotid patency status after thrombolysis. However, at 30 micro g/kg and more, YM-254890 decreased systemic blood pressure. These results suggest that YM-254890 may be effective for treating G(q)-mediated diseases, and that YM-254890 is a useful tool for investigating the biological roles of G(q/11).