The mathematical model of solute and water removal is devised in order to determine the optimum sodium concentration in dialysate and substitution fluid. This model involves the movement of water and solutes between each body compartment. The theoretical results agree well with the clinical data. The results indicate that the change of intracellular volume during hemopurification therapy is independent of the urea concentration gradients between the intracellular and extracellular compartments, and that the effective factors required to determine the optimum sodium concentration in dialysate and substitution fluid are pre-treatment plasma concentration, effective osmolarity, volume of body fluid and body fluid removal. The effects of solute and body fluid removal on the plasma volume changes are also analysed.