The volume of distribution is the apparent volume in which the drug can be distributed in. It is the factor that relates the amount of drug in the body to the concentration of the drug in the sampling site. It is not an actual volume, but it is a hypothetical volume based on the drug distribution behavior.

In the two-compartment pharmacokinetic model there are more than one volume of distribution for the drug. Initially after iv drug administration, the drug is distributed only in the central compartment. Then the drug is distributed in both the central and peripheral compartments.

During the elimination phase the drug volume of distribution is equal to Vd_β, and during the steady state the drug volume of distribution is equal to Vd_ss.