Abstract: An LC-MS/MS method was developed for the simultaneous determination of fosaprepitant and aprepitant in human plasma, and applied to a pharmacokinetic study of 150 mg fosaprepitant dimeglumine injection to 12 Chinese healthy volunteers. The analytes and internal standards were extracted from plasma by protein precipitation with acetonitrile and separated on a Cortex C18+ (50 mm×2.1 mm, 2.7 μm) column using a gradient elution procedure. Mass spectrometry was performed in negative MRM mode, and parent-to-produce transitions were as follows:m/z 613.1→78.9 for fosaprepitant, m/z 617.0→78.9 for d₄-fosaprepitant, m/z 533.2→275.1 for aprepitant and m/z 537.2→279.1 for d₄-aprepitant. Plasma sample was basified to stabilize fosaprepitant. The standard curves were demonstrated to be liner in the range of 15.0 to 6 000 ng·mL^-1 for fosaprepitant and 10.0 to 4 000 ng·mL^-1 for aprepitant. The intra-day precisions and inter-day precisions and accuracy were within the acceptable limits for all concentrations.