Summary.Heavy water is injected into the vein of rabbits and blood samples taken at intervals from the artery. The density of the water prepared from the blood samples is determined and, from the density difference between the injected heavy water and the blood water, the extent of dilution, which the heavy water molecules experienced in the body at different times, calculated.As soon as a1/2min. after the injection, a dilution of the heavy Water by an amount of body water corresponding in volume to about that of the extracellular space of the body, is found. This very rapid rate of dilution is followed by a somewhat slower dilution process, in which presumably the cellular water participates.After the lapse of less than1/2hour, the heavy water molecules are evenly distributed over almost the total body water.After the lapse of 39 days, only about1/2of the heavy water injected is still present in the body.There is no reason to assume that the heavy water (mainly DHO) molecules show a markedly different behaviour from that of the normal water (H2O) molecules present in the body, and we have, therefore, to conclude that within about1/2min. a sufficient flow of water takes place through the capillary walls to lead to an almost perfect mixing of the blood and the interspace water. An analogous interaction between cellular and extracellular water takes less than1/2hour.