While pharmacokinetics is the study of drug concentrations in particular compartments as a function of time, pharmacodynamics is concerned with drug effects. Because most drugs are small molecules that act through binding to an enzyme or to a macromolecular receptor, pharmacodynamics is closely related to the science of enzyme kinetics, which relates the rate of an enzyme-catalyzed reaction to the concentrations of its reactants (substrates, activators, inhibitors, and the enzyme itself). A drug effect may be as simple as inhibition of an enzyme, or it may involve interaction with a more complex system, as when, for example, an ion-channel blocker causes a lowering of blood pressure. Pharmacodynamics is thus concerned with drug effects over a range of levels of complexity. The general scope of the subject is outlined in a chapter of Goodman and Gilman (Gilman et al., 1980). These authors emphasize three basic tenets of pharmacodynamics: the central position of the dose-response relationship in pharmacology, the importance of understanding a drug's mechanism of action, and the concept of selectivity. As in pharmacokinetics, pharmacodynamic parameters may be scaled across species, either by allometry or by physiological modeling (Mordenti and Green, 1991), so that we may attempt to use preclinical pharmacodynamic data to predict the response of humans to a developmental drug. A recent work that reviews the role of pharmacodynamics in clinical drug development is Yacobi et al. (1993).