The changes of the distribution of body fluid following acute isotonic dialyzation were studied from the results of continuous monitoring of extracellular fluid volume and physiological parameters. An indicator of extracellular space, ⁵¹Cr-EDTA, was injected into spleno-nephrectomized dogs. After the equilibrium of tracer dilution was attained, 10ml/kg of plasma water was isotonically withdrawn by means of a dialyzer of hollow fiber type. The volumes of extracellular fluid (ECF) and plasma (PV) were continuously monitored for 80min and plasma osmolality was measured at 10min intervals. At the 50-60th min after the fluid modification, the reduction of PV was only 3.8ml/kg and that of ECF was 4.2ml/kg. The continuous profile of ECF change showed a significant mobilization of water from intracellular fluid (ICF) soon after the dialyzation. It is concluded that, in the state of hypovolemia, plasma fluid is replenished with the transvascular fluid absorption from interstitial space and that, concomitantly, the reduction of ISF is restituted with the fluid mobilization from ICF into extracellular space within the early stage of 1hr. Good linear correlation was found between the amount of mobilized water from ICF and the increment of plasma osmolality. An increase in osmotic force was considered the mechanism which caused the fluid shift. These findings suggest that the change in plasma osmolality is a good predictor of mobilized volume from ICF in hypovolemia. The effects of inverse fluid modification, i.e., isotonic infusion, were also compared.