Metabolite profiling and identification studies provide information on the nature and the number of metabolites formed by a drug compound in specific species and test systems. The molecular weight and elemental composition of metabolites are determined along with their proposed structures. During the drug discovery phase, such information is used to decide whether chemical structures should be further modified. More importantly, it helps evaluate the presence of pharmacologically active or chemically reactive metabolites with potential toxicity and whether they are found uniquely or disproportionately in humans versus other species. Hence, it is encouraged to perform in vivo metabolite profiling and identification studies in nonclinical test species early during preclinical development.