Abstract-Clopidogrel is an effective new antiplatelet agent useful for the treatment of ischemic cerebrovascular, cardiac, and peripheral arterial disease. However, the mechanism of clopidogrel action is not well understood, although it is known to inhibit ADP-evoked platelet aggregation. In the current study, the effect of clopidogrel on recently identified human platelet ADP receptors and their signaling pathways was investigated by using platelets from clopidogrel-treated subjects, 6 healthy volunteers (2 females and 4 males) who received 75 mg of clopidogrel daily for 7 days. Blood was taken and various platelet receptor signaling pathways were analyzed before treatment, after 7 days of medication, and 4 weeks after treatment had ceased. Platelet tests included the analysis of aggregation, rapid calcium influx, calcium mobilization from intracellular stores, adenylyl cyclase, and phosphorylation of vasodilator-stimulated phosphoprotein (VASP). The data indicate that clopidogrel does not affect those platelet ADP receptors coupled to cation influx (P2X1 ADP receptors) or calcium mobilization (P2Y1 ADP receptors). In contrast, clopidogrel treatment specifically impairs the ADP receptor coupled to G i /adenylyl cyclase (P2Y AC ADP receptors). Clopidogrel abolishes the inhibitory P2Y AC receptor-mediated ADP effects on prostaglandin E 1 -stimulated, cAMP-dependent phosphorylation of VASP without affecting epinephrine, thrombin, and thromboxane signaling. VASP phosphorylation is known to be closely correlated with the inhibition of platelet and fibrinogen receptor (glycoprotein IIb/IIIa) activation. Therefore, inhibition of the platelet P2Y AC ADP receptor and its intracellular signaling, including decreased VASP phosphorylation, is suggested as a molecular mechanism of clopidogrel action. Key Words: platelet inhibition Ⅲ purinergic receptors Ⅲ vasodilator-stimulated phosphoprotein I ncreased platelet activation and aggregation are central to the pathophysiology of acute and chronic arterial vascular diseases. This concept has gained broad acceptance since platelet inhibitors have been proven as effective agents for the treatment of both chronic and acute diseases of the arterial vessel wall. [1][2][3][4][5][6] Platelets are activated by numerous agents and conditions, but ADP is thought to play a key role in the development of arterial thrombosis. 7,8 Recently, long-term administration of clopidogrel to patients with atherosclerotic vascular disease was shown to be more effective than aspirin in reducing the combined risk of ischemic stroke, myocardial infarction, and vascular death. 9 Clopidogrel and the chemically related ticlopidine are thienopyridines that selectively and specifically interfere with ADP-mediated platelet activation. 5,10 In contrast to ticlopidine, which may cause neutropenia, clopidogrel appears to be a safe and well-tolerated drug. 9 Thienopyridines are inactive in-vitro, require in vivo metabolism, and cause an irreversible inhibition of platelet function. However, the mechanism of thieno...