Pharmacokinetics may be defined as what the body does to a drug. It deals with the absorption, distribution, and elimination of drugs but also has utility in evaluating the time course of environmental (exogenous) toxicologic agents as well as endogenous compounds. An understanding of 4 fundamental pharmacokinetic parameters will give the toxicologic pathologist a strong basis from which to appreciate how pharmacokinetics may be useful. These parameters are clearance, volume of distribution, half-life, and bioavailability.