sible for the remaining reduction of NaCl reabsorption are less clearly understood but micropuncture studies have demonstrated that this occurs in the loops of Henle of deep medullary nephrons and may be coupled to washout of the medullary solute gradient that affects the passive reabsorption of sodium in this portion of the nephron. 3 Some investigators have found similar relationships between renal perfusion pressure and MBF in dogs, 22 whereas others have not. 23 However, the results of numerous studies in rats have been consistent, and although influenced by the state of hydration these results indicate that changes of medullary blood flow provide the signal for the pressure-natriuresis response and are the vital link to the long-term control of sodium excretion and arterial blood pressure.