Oxidant stress has been implicated in a wide variety of disease processes. One method to quantify oxidative injury is to measure lipid peroxidation. Quantification of a group of prostaglandin F_2α-like compounds derived from the nonezymatic oxidation of arachidonic acid, termed the F₂-isoprostanes (F₂-IsoPs), provides an accurate assessment of oxidative stress both in vitro and in vivo. In fact, in a recent independent study sponsored by the National Institutes of Health (NIH), F₂-IsoPs were shown to be the most reliable index of in vivo oxidant stress when compared against other well known biomarkers. This protocol details our laboratory's method to quantify F₂-IsoPs in biological fluids and tissues using gas chromatography-mass spectrometry (GC-MS). This procedure can be completed for 12–15 samples in 6–8 h.