The main objective of this chapter is to examine the kinetic differences between preformed and generated metabolites and highlight potential complications associated with the approaches in risk assessment of drug metabolites by dosing preformed metabolites. To illustrate the kinetic differences between preformed and generated metabolites, theoretical considerations and specific examples are presented. The kinetic behavior of several preformed metabolites given exogenously is shown to differ significantly from that of the corresponding metabolites generated endogenously from the parent compound in vivo. Demonstrated differences exist in the accumulation of the preformed versus generated metabolites in specific tissues, and in sequential metabolism to downstream products. In such cases, a “bridging study” performed by dosing the preformed metabolite in animals would fail to characterize the true toxicological contribution of the metabolite generated from its parent and complicate the toxicity evaluation of the metabolite. It is recommended that the kinetic behavior of a metabolite generated in vivo versus that given exogenously be assessed before conducting the so-called bridging study. Given recent advancements in transgenic animal models, it is conceivable that sometime in the future humanized mouse lines expressing specific drug transporters and metabolizing enzymes may serve as valuable tools for risk assessment of drug metabolites that are disproportionately higher in humans than in animal species used for toxicity studies.