The functional characteristics of the ascending limb of Henle's loop were examined during hypotonic saline infusion by measuring solutefree water clearance (C(H2O)) at varying rates of solute delivery. The influence of expansion of extracellular volume was studied by comparing C(H2O) during hypotonic saline diuresis in normal dogs with dogs whose extracellular volume had been expanded acutely by saline infusions or chronically by the administration of deoxycorticosterone acetate and salt. In normal animals hypotonic saline infusions greatly increased urine flow (V) and C(H2O) without appreciably augmenting osmolar clearance (C(osm)). C(H2O) was, therefore, analyzed as a function of V, rather than C(osm), since V was the best estimate of delivery of filtrate to the diluting segment. C(H2O) increased as a linear function of V without any evidence of saturation.The validity of interpreting increases in C(H2O) and V as indications of increased sodium reabsorption and delivery was reinforced by tissue studies that disclosed a rise in papillary osmolality with rising urine flows. The observed increase in C(H2O) and V could not, therefore, be due to a decrease in back diffusion of solute-free water as a result of a diminished osmotic driving force, but probably represented increased formation consequent to augmented delivery as a result of decreased fractional reabsorption in the proximal tubule. In animals whose extracellular volume was acutely or chronically overexpanded before the infusion of hypotonic saline, sodium excretion was greater, and C(H2O) less, at any given V. Although the curve relating C(H2O) to V was flatter than in the control group, no tubular maximum was observed. The diminished C(H2O) in this group was interpreted to mean that massive expansion of extracellular volume inhibits sodium reabsorption in the ascending limb of Henle's loop.