Introduction The happy facility which enables the organism to vary, over rather wide ranges, the relative proportions of urinary water and solute is a phenomenon of manifest teleologic advantage and constitutes perhaps the major line of defense with regard to homeostasis of volume and tonicity of the body fluids. The osmotic pressure of the extracellular fluids, and presumably of the cells they bathe, is punctiliously guarded at values of approximately 280 to 300 mOsm. per liter.* The cellular milieu of the human kidney, however, must somehow cope with the fluid contents within its tubules, which between the extremes of maximum diuresis and antidiuresis ranges from under 50 to more than 1,300 mOsm. per liter, that is, roughly one-sixth to over four times the osmotic pressure of the plasma. The kidney of other mammals may perform even more spectacularly: the urine within the collecting tubules of certain desert rodents that subsist