The thick ascending limb of the loop of Henle (TAL segment) is responsible for the generation of concentrated urine in antidiuresis and dilute urine in water diuresis. This nephron segment dilutes the lumen fluid and generates a hypertonic interstitium, because it is able to reabsorb NaCl and prevent a corresponding HZ0 reabsorption by its water impermeability. In antidiuresis, the interstitial hypertonicity is used to withdraw water from the collecting ducts, and a highly concentrated urine is formed. In water diuresis, the collecting ducts are water impermeable, and the diluted tubule fluid is excreted in urine. The functional importance of the TAL segment became apparent when it was recognized that the early distal tubule fluid is dilute with respect to plasma (163). An analysis of the functional characteristics, however, could not be performed by the micropuncture methodology, because the TAL segment hides its secrets beneath the kidney surface.