This paper aims to provide a scientifically based perspective on issues surrounding the proposed toxicology testing of synthetic drug metabolites as a means of ensuring adequate nonclinical safety evaluation of drug candidates that generate metabolites considered either to be unique to humans or are present at much higher levels in humans than in preclinical species. We put forward a number of theoretical considerations and present several specific examples where the kinetic behavior of a preformed metabolite given to animals or humans differs from that of the corresponding metabolite generated endogenously from its parent. The potential ramifications of this phenomenon are that the results of toxicity testing of the preformed metabolite may be misleading and fail to characterize the true toxicological contribution of the metabolite when formed from the parent. It is anticipated that such complications would be evident in situations where (a) differences exist in the accumulation of the preformed versus generated metabolites in specific tissues, and (b) the metabolite undergoes sequential metabolism to a downstream product that is toxic, leading to differences in tissue-specific toxicity. Owing to the complex nature of this subject, there is a need to treat drug metabolite issues in safety assessment on a case-by-case basis, in which a knowledge of metabolite kinetics is employed to validate experimental paradigms that entail administration of preformed metabolites to animal models.