We simulated the change in plasma volume by hemodialysis by combining two models for transcapillary and transcellular fluid exchange. Model predictions of the plasma volume change were found to be in good agreement with the measured values in five patients who were studied during 4-hour hemodialysis at a constant ultrafiltration rate of 0.5 L/hr using three different dialysate sodium concentrations: 7% below, 7% above, and equal to the predialytic serum concentration. The measurements and model predictions indicate that the decrease in plasma volume is smaller in high sodium dialysis than in normal and low sodium dialyses. Because model predictions were consistent with measurements, this model should be useful in clinical practice to quantitatively analyze the change in plasma volume during hemodialysis and to relate it with hypovolemic hypotension.