Aniline, widely used as an intermediate in the synthesis of dye, is also used in the manufacture of pharmaceuticals, photographic developers, shoe polish, and other common substances. Exposure to aniline is toxic because it produces methemoglobin. Aniline levels are usually not measured in serum; in humans, blood methemoglobin levels are often measured as an index of exposure to aniline. In this article, we describe a method for the identification and the quantification of aniline by gas chromatography/mass spectrometry (GC/MS) after its extraction from human serum and derivatization with 4-carbethoxyhexafluorobutyryl chloride. Aniline, as well as the internal standard N-methyl aniline, was extracted from alkaline serum using chloroform. Aniline and the internal standard were derivatized with 50 microL of 4-carbethoxyhexafluorobutyryl chloride. After evaporating the excess derivatizing reagent, the residue was reconstituted in 50 microL of ethyl acetate and injected into the GC/MS. A positive identification of derivatized aniline can be made from the strong molecular ion at m/z 343. Similarly, derivatized internal standard showed a strong molecular ion at m/z 357. The within-run and between-run precisions of the assay were 3.8% and 5.8%, respectively, at an aniline concentration of 5 mg/L. The assay was linear for serum aniline concentrations of 0.5 mg/L to 25.0 mg/L. The detection limit was 0.1 mg/L. The assay was not affected by lipemia, hemolysis, or high bilirubin concentration in serum, and the assay was applicable to whole blood. We also fed mice (C57bl/6) with various concentrations of aniline and measured methemoglobin and blood concentrations of aniline. The methemoglobin percentage and aniline concentrations in blood increased with increasing aniline doses.