Vasopressin-related alterations of sodium reabsorption by the loop of Henle. Recollection micropuncture experiments were performed in dogs to determine distal tubular fluid sodium concentration, osmolality and water reabsorption during three states of hydration: 1) spontaneous antidiuresis, 2) water diuresis and ECF volume expansion, and 3) vasopressin-induced antidiuresis during sustained ECF volume expansion. Distal TF/PNa and TF/POsm ratios decreased in parallel during water diuresis. During vasopressin infusion and an established hypertonic urine, TF/PNa and TF/POsm returned to values observed during spontaneous antidiuresis. These parallel and reversible changes in distal sodium concentration were not associated with appropriate changes in water reabsorption, as indicated by the constancy of recollection TF/P inulin ratios, by either the proximal nephron alone or by the entire nephron to the distal site of collection. The results suggest that the water diuresis-associated reduction in distal sodium concentration is due to increased net sodium reabsorption in the loop of Henle; vasopressin abolishes this increase in sodium reabsorption in both normovolemic and expanded dogs. Alterations in medullary blood flow, induced directly or indirectly by vasopressin, may explain the observed changes in distal tubule sodium concentration.