Clopidogrel is a new thienopyridine derivative similar to ticlopidine, which inhibits adenosine diphosphate-induced platelet aggregation. The in vitro effects of clopidogrel on shear-induced platelet activation and coagulation were assessed after oral administration to rats, by subjecting non-anticoagulated blood to haemostatometry. Clopidogrel significantly inhibited shear-induced platelet activation and coagulation 2 h after administration at doses of 7.5 and 15 mg/kg. Both ticlopidine (200 mg/kg) and aspirin (200 mg/kg) inhibited shear-induced platelet activation, but not coagulation. The peak inhibition of plaetelet activation by clopidogrel occurred 2 h after oral administration, but significant inhibition persisted even after 24 h. These results suggest that clopidogrel could be a more potent antithrombotic agent than ticlopidine or aspirin, and also that ADP plays an important role in shear-induced platelet activation.