Among the models describing respiratory mechanics none has been published with the characteristics of two lung compartments including the viscoelastic properties. We used such a model to describe the inspiratory compartmental volume distribution under homogeneous and inhomogeneous conditions. The present mathematical model was tested against actual data and proven accurate. The volume distribution was studied using data from normal subjects and from patients with COPD and ARDS. In a normal lung, changes in viscoelastic constants in one compartment can modify substantially the volume distribution diverting more or less gas to the other compartment. In diseased compartments, the increase of viscoelasticity increased the difference between the compartments and the opposite was true in the less affected compartment. In conclusion, the viscoelastic properties are of paramount importance in determining gas distribution in normal and sick lungs.