The consensus of most published opinion is that most mammalian cells conduct their fluid exchanges in osmotic equilibrium, without active transport of water. External osmotic pressure is important for the regulation of cell volume chiefly because it determines how much water shall be associated with a given quantity of intracellular solute. The cell s own metabolism is equally important, because this determines how much intracellular solute there shall be. In the intact animal with metabolism proceeding steadily, extracellular osmolarity controls the water balance of the cells. Hence thirst and water diuresis, which rather precisely guard against excessive or deficient levels of extracellular osmolarity, may be regarded as mechanisms controlling the volume of intracellular fluid. Since they do this by stabilizing an extracellular osmolarity which is mostly due to sodium salts, they also set the stage for the regulation of extracellular fluid volume by adjustments of the renal excretion of sodium. For so long as its osmolarity is held constant, the volume of extracellular fluid must be proportional to the amount of sodium which it contains, and this depends upon how much of the daily intake the kidneys retain. Hence the kidneys, directed in ways which largely remain to be elucidated are able to regulate the volume of water inside the cells by controlling the excretion of water, and the volume outside the cells by controlling the excretion of sodium. Thus the organization of intracellular as well as of extracellular fluids, together with the exchanges both between compartments within the body and between the body and the external environment, may be described to a remarkable extent, in Gamble''s happy phrase, as a "companionship of water and electrolytes".