Atrazine (ATZ), one of the most widely used herbicides worldwide, has been the subject of several scientific studies associated with its human and ecological risks. In order to study atrazine's toxicity, the formation of its metabolites and the result of their exposure must be assessed. This relies on our ability to detect and identify all of atrazine's metabolites; however, no previous untargeted screening method has reported the detection of all known metabolites and glutathione conjugates at once. In this study, a compound-specific, postacquisition metabolic screening method was employed following a generic HPLC separation coupled with high resolution time-of-flight mass spectrometry (TOF-MS) to detect Phase I metabolites and glutathione conjugates generated by in vitro human liver microsomal incubations. Our method was designed to be unbiased and applicable to a wide variety of compounds since methods that can detect a broad range of metabolites with high sensitivity are of great importance for many types of experiments requiring thorough metabolite screening. On the basis of incubations with atrazine and three closely related analogues (simazine, propazine, and cyanazine), we have proposed a new Phase I metabolism scheme. All known Phase I transformations of atrazine were successfully detected, as well as a new N-oxidation product. Novel reactive metabolites were also detected as well as their glutathione conjugates. These newly detected species were produced via imine formation on the N-ethyl group, a biotransformation not previously observed for atrazine or its analogues.