In pharmacokinetic modeling it is common to use compartmental structures to describe the disposition of a drug in the blood or plasma. Typically, a linear multicompartment mammillary model is equated with the multiexponential equation derived from observing the decay of the plasma drug concentration following an intravascular injection. Classically, the mammillary models are constructed so that the concentrations in each of the compartments are equal at steady state, the apparent volume of distribution at steady state is equal to the sum of the individual compartment volumes, and the apparent volume of each peripheral compartment is equal to the ratio of its intercompartmental rate constants times the central compartment volume. On the basis of what can be measured in the plasma, however, it is equally valid to assume that the sizes of the peripheral compartment volumes are equal to the central compartment volume and that the steady-state concentration in each peripheral compartment is equal to the ratio of its intercompartmental rate constants times the concentration in the central compartment. In fact, these are but two of an infinite number of interpretations of the peripheral compartment volumes.