Traditionally, only circulating concentrations of parent drug have been measured in the rodent and nonrodent test species used for drug safety assessments and served as an index of systemic exposure for comparisons to human exposures. Circulating concentrations of metabolites have generally only been measured in specialized circumstances (e.g., parent compound was extensively metabolized). Measurement of only the parent compound is usually sufficient when the metabolite profile in humans is similar to that in at least one of the animal species used in the nonclinical safety assessment. However, it is possible that metabolites formed in humans might not be present in the rodent and nonrodent test species used for drug safety assessments or the metabolites are formed at disproportionately higher concentrations in humans than in the animal test species. Generally, metabolites identified only in human plasma or metabolites present at disproportionately higher concentrations in humans than in any of the animal test species should be considered for safety assessment. The Center for Drug Evaluation and Research (CDER) published a Guidance for Industry on Safety Testing of Drug Metabolites that provides current thinking within CDER on the nonclinical safety assessment of human drug metabolites derived from drug products. The CDER guidance defines human metabolites that can raise a safety concern as those formed at greater than 10% of parent drug systemic exposure at a steady state. By contrast, the more recent International Conference on Harmonization: Guideline on Nonclinical Safety Studies for the Conduct of Human Clinical Trials and Marketing Authorization for Pharmaceuticals (ICH M3[R2]) describes the threshold as 10% of total drug-related exposure. Where they differ, the ICH guidance supersedes the CDER Guidance. The purpose of this article is to provide a perspective on the important details of these guidances from a regulatory review standpoint, as well as discuss some concerns that have arisen from the regulated industry regarding the CDER guidance. Such issues include parent drug that is extensively metabolized, metabolism by intestinal bacteria and metabolites formed by nonclinical test species but not humans.