Two fast and sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS)-based bioanalytical assays were developed and validated to quantify the active and three inactive metabolites of prasugrel. Prasugrel is a novel thienopyridine prodrug that is metabolized to the pharmacologically active metabolite in addition to three inactive metabolites, which directly relate to the formation and elimination of the active metabolite. After extraction and separation, the analytes were detected and quantified using a triple quadrupole mass spectrometer using positive electrospray ionization. The validated concentration range for the inactive metabolites assay was from 1 to 500 ng/mL for each of the three analytes. Additionally, a 5× dilution factor was validated. The interday accuracy ranged from −10.5% to 12.5% and the precision ranged from 2.4% to 6.6% for all three analytes. All results showed accuracy and precision within ±20% at the lower limit of quantification and ±15% at other levels. The validated concentration range for the active metabolite assay was from 0.5 to 250 ng/mL. Additionally, a 10× dilution factor was validated. The interbatch accuracy ranged from −7.00% to 5.98%, while the precision ranged from 0.98% to 3.39%. Derivatization of the active metabolite in blood with 2-bromo-3'-methoxyacetophenone immediately after collection was essential to ensure the stability of the metabolite during sample processing and storage. These methods have been applied to determine the concentrations of the active and inactive metabolites of prasugrel in human plasma. Copyright © 2006 John Wiley & Sons, Ltd.