The principal objective of toxicokinetic studies is to assess systemic exposure of the toxicity species to test compound during nonclinical toxicity studies by generation of pharmacokinetic data on the rate, extent, and duration of exposure. Toxicokinetics are normally integrated within the toxicity studies (concomitant); however, there are a number of circumstances when prospective, retrospective, or other ancillary types of toxicokinetic studies are performed in support of toxicity studies. The need for retrospective toxicokinetic studies arises when the original toxicity study was conducted with insufficient plasma concentration determinations, especially when pilot toxicity studies show lack of toxicity or unusual organ toxicities. Examples of ancillary toxicokinetic studies from the Drug Metabolism and Drug Safety departments at The R. W. Johnson Pharmaceutical Research Institute and from the scientific literature are provided that highlight the challenges associated with these alternate approaches. Each example includes a discussion of the rationale for the supportive toxicokinetic study, the specific experimental designs, the data interpretation, and the usefulness of the toxicokinetic data in decision-making. The ancillary toxicokinetic studies cited provided important information on establishing systemic exposure during long-term toxicity studies, allowing comparisons to human exposures, elucidating compound-related toxicities, and explaining lack of toxicity due to autoinduction and resulting low levels of parent compound.