A potent anticoagulant activity of Boophilus microplus crude saliva has been previously identified (Horn et al., Arch. Biochem. Biophys. 384: 68–73, 2000). In this work we compared the in vivo effect of saliva (full engorgement) and extract of glands (partially engorgement) from B. microplus. In order to determine whether B. microplus saliva and extract of glands could exert its antithrombotic effect, we used a thrombosis model that combines stasis and hypercoagulability in rats. A control group received tissue thromboplastin doses of 3 mg/kg and the mean thrombus weight was 9.2 ± 0.1 mg (n = 8). Intravenous administration (I.V.) of either saliva or extract of glands (400 and 81 mg, respectively) decreased thrombus weight by 81% and 100%, respectively. The antithrombotic action of saliva (400 mg) showed a time-dependent pattern, showing 81%, 92% and 26% of inhibition after 5, 30 and 60 min of administration, respectively. The ex vivo clotting tests were evaluated using PT, aPTT and TT tests. No significant changes were observed as compared to control animals treated with saline. The bleeding effect was assessed based on blood loss from a rat cut-tail, after I.V. administration of the samples. Both materials (400 mg) showed a increase of ~2.5-fold in hemorrhagic effect when compared to control values. Altogether, our data indicate that B. microplus saliva and extract of glands display potent antithrombotic action.