A novel, sensitive, and specific method is presented for the quantification of endogenous 3-nitrotyrosine in rat plasma based on isotope dilution liquid chromatography-electrospray ionization tandem mass spectrometry, using 3-nitro-2,5,6-d 3 -L-tyrosine as an internal standard. The extraction and cleanup method entails three major steps: protein precipitation, solidphase extraction with an aminopropyl cartridge, followed by derivatization of 3-nitrotyrosine to the corresponding butyl ester. The analysis of the stable butyl ester derivative circumvented matrix interferences, which were encountered on the analysis of the nonderivatized analyte in plasma, and thus significantly improved sensitivity. The mass spectral acquisition of 3-nitrotyrosine butyl ester was done in the positive ion mode using selected reaction monitoring of two specific transitions. The response was linear over the concentration range 1.4 -28.5 nM, and the recoveries of spiked 3-nitrotyrosine in rat plasma exceeded 75%. The detection and quantification limits of 3-nitrotyrosine in rat plasma (165 L equivalent injected) approached 0.43 and 1.4 nM (0.07 and 0.23 pmol, on column), respectively. This study also addresses the potential artifactual formation of 3-nitrotyrosine, which may lead to an overestimation of the background levels of the biomarker. Solid-phase extraction of 3-nitrotyrosine was required prior to esterification to avoid artifactual nitration of tyrosine. In this context, analysis of eight rat plasma samples showed quantifiable levels in only four of the samples of the order of 1.4 -1.5 nM.