Sodium excretion and proximal tubule function were studied in rats with an aorta-to-vena cava (A-V) fistula using the techniques of micropuncture. 15–28 days after ereation of the fistula, 40% of the rats had developed peripheral edema and ascites. Control rats excreted significantly more sodium in the urine than did the A-V fistula rats. Clearances of inulin and PAH, intrarenal distribution of filtration, and filtration fraction were wholly comparable for the two experimental groups. Also, sodium reabsorption by the proximal tubule was not different for the two groups during either hydropenia or extracellular volume expansion. Transit time through the loop of Henle was prolonged in the A-V rats. The data suggest that an enhanced reabsorption in a distal nephron segment – possibly in the loop of Henle – must play a significant role in sustained salt retention and edema formation of A-V rats. Proximal reabsorption was increased only in ‘acute’ A-V rats (2–6 days after the operation). This alteration of proximal reabsorption does not appear relevant to the chronic state of sodium retention and may be mediated by extracellular fluid volume contraction due to creation of