The antithrombotic activity of a 2-kDa heparin fragment was studied in a rat model of common carotid artery thrombosis that causes a completely occlusive thrombus with cessation of the blood flow within 10-15 min. The compound reduced thrombus formation in a dose-dependent manner, starting from an intravenous dose of 5 mg kg(-1). A dose of 20 mg kg(-1) completely prevented thrombus formation and apparently induced the almost complete lysis of the already formed occlusive thrombus. At none of the doses used did the compound cause increased bleeding or the formation of haematomas. The present results indicate that low molecular weight heparins, which have an established, highly beneficial effect in venous thromboembolism, are also highly effective in an animal model of arterial thrombosis.