A network thermodynamic model has been devised to describe the coupled movement of water and glycerol molecules within the kidney during perfusion with solutions of time-varying glycerol and impermeant comcentrations. Both flows are governed by a combination of the individual osmotic potential differences between compartments, the viscoelastic behavior of the tissue, and the momentum transferred between the two flows. The system is modelled using the bond graph technique and simulated with the ENPORT7 package (Rosencode Associates, 1993).