Glutathione (GSH, γ-glutamyl-cysteinyl-glycine) is the most abundant intracellular dicarboxylic tripeptide with multiple physiological roles. GSH and its symmetric disulfide GSSG co-exist in biological samples. The GSH-to-GSSG molar ratio is high within cells and low in extracellular compartments. It is widely used as a biomarker of oxidative stress. Here, we report on a novel two-step derivatization procedure which allows for the specific quantitative measurement of GSH by gas chromatography-mass spectrometry (GC-MS) in the electron-capture negative-ion chemical ionization mode. The method is based on the derivatization of GSH first with pentafluoropropionic anhydride in ethyl acetate (30 min, 65 °C) and subsequently with 2 M HCl/CHOH (30 min, 80 °C). Esterification in 2 M HCl/CDOD is used for the in situ synthesis of deuterium-labelled GSH for use as internal standard. Quantification is performed by selected-ion monitoring of m/z 557 for GSH and m/z 560 for the internal standard (1 mM). Linear regression analysis between the peak area ratio of m/z 557 to m/z 560 (y) and the GSH concentration (x; range, 0-1 mM) resulted in the regression equations y = 0.02 + 0.62x (r = 0.976) in the absence and y = 0.02 + 0.63x (r = 0.997) in the presence of GSSG (range, 0-1 mM), underlying the linearity of the method for GSH and suggesting lack of interference by GSSG. γ-Glu-Cys and Cys-Gly also did not interfere. The GC-MS method demonstrated that in human hemolysate, nitrite at toxicological concentrations (range, 0-15 mM) depletes erythrocytic GSH. For the GC-MS analysis of the GSH tripeptide analogue ophthalmic acid, which lacks a Cys moiety, the derivatization order was HCl/CHOH followed by pentafluoropropionic anhydride in ethyl acetate.