After drug administration, the body behaves like the beaker in the previous example. The only difference is that in addition to drug distribution to different tissues, the body can also eliminate the drug.

When drug distribution to the tissues is fast and rapid equilibrium is achieved between all tissues, the drug follows one-compartment pharmacokinetic model. A plot of the plasma conc-time profile is linear on a semilog scale.

When the drug is distributed slowly to the tissues, the drug follows two-compartment pharmacokinetic model. A plot of the plasma conc-time profile on a semilog scale consists of two parts, a distribution phase and an elimination phase.