Background The aim of this study was to compare rate of onset, magnitude, and consistency of platelet inhibition after administration of prasugrel or clopidogrel and to relate platelet inhibition to systemic exposure to each active metabolite. Thienopyridines are prodrugs, metabolized in vivo to active metabolites that inhibit the platelet P2Y12 adenosine diphosphate (ADP) receptor. Methods This was an open-label, 2-way, crossover study that randomized healthy subjects (n = 68) to an oral loading dose (LD) of prasugrel 60 mg or clopidogrel 300 mg. Platelet aggregation response to 5 and 20 μmol/L of ADP was measured by turbidometric aggregometry. Plasma concentrations of the active metabolites of prasugrel and clopidogrel were quantified by liquid chromatography with tandem mass spectrometry detection methods. Results Inhibition of platelet aggregation (IPA) after prasugrel was significantly higher ( P < .01) than that after clopidogrel from 15 minutes through 24 hours (5 μmol/L ADP) and from 30 minutes through 24 hours (20 μmol/L ADP). For 20 μmol/L ADP, the median time to reach ≥20% IPA was 30 minutes for prasugrel and 1.5 hours for clopidogrel ( P < .001). The maximum IPA was 84.1% ± 9.5% with prasugrel versus 48.9% ± 27.0% with clopidogrel for 5 μmol/L ADP and 78.8% ± 9.2% versus 35.0% ± 24.5%, respectively, for 20 μmol/L ADP ( P < .001). Response to prasugrel was more consistent compared to clopidogrel ( P < .01). The lower IPA response to clopidogrel was associated with lower plasma concentrations of its active metabolite ( P < .001). Conclusions Prasugrel 60 mg LD results in more rapid, potent, and consistent inhibition of platelet function than clopidogrel 300 mg LD. Lower IPA responses to clopidogrel were associated with lower concentrations of its active metabolite.