Anatomical and physiological differences are the main factors that cause discrepancy between human and animals with respect to the pharmacokinetics of xenobiotics. Therefore, insight into the consequences of interspecies differences in anatomy and physiology on the pharmacokinetics of various classes of compounds may lead to improved animal-human extrapolation. Dog, monkey, mouse, (mini)pig, rabbit, and rat were selected because they are commonly used in studies on pharmacokinetics and toxicology of xenobiotics for the prediction of the human pharmacokinetics and toxicity. Otherwise, the pig was chosen since general statements can be found in literature, that pigs are quite comparable to humans. Moreover, insight into intraspecies differences may be important, since large differences between subspecies can exist.