Please note that when modelling pharmacokinetic data, 3 compartments is possible. In this case the plasma concentration versus time is fitted with Cp(t) = Ae^(at)+Be^(bt)+Ce^(ct). A 3rd compartment may be indicative of a very slow elimination phase. The highest slope is the distribution phase. The slower phase is the elimination phase. If there are 3 compartments, then that would be the phase with the smallest slope value. Note, that if there is a 3rd compartment then drug accumulation can occur with multiple drug doses or long IV drug infusions. Note that a 3rd compartment can be missed depending on the time points taken. Some data can fit both a 2-compartment and 3-compartment model. The simplest model that is sufficient should be used. Higher exponential curve fitting (triexponential) will always fit better than one of fewer exponentials (i.e., monoexponential).