1. Distal blockade by simultaneous administration of ethacrynic acid and chlorothiazide was performed in healthy subjects whose extracellular fluid was expanded by iso-osmotic sodium chloride or bicarbonate. The results obtained were compared with those from non-expanded subjects (Ardaillou & Fillastre, 1969). 2. By this technique urine approximates in composition to proximal tubular fluid and may be used to provide information on its composition. As in other mammals, U Na /P Na and U osm /P osm were close to 1, whatever the experimental conditions. U Cl /P Cl was always higher and U HCO3 /P HCO3 always less than 1. 3. Extracellular fluid expansion with sodium chloride depresses water and sodium reabsorption as shown by the increase of tubular fluid and the diminution of U Inul /P Inul and T Na /GFR × P Na where T Na and GFR × P Na are respectively the amounts of sodium reabsorbed and filtered per min. It also decreases bicarbonate proximal Tm. The influence on bicarbonate reabsorption is more marked in alkali-loaded than in acid-loaded subjects. 4. Extracellular fluid expansion with iso-osmotic sodium bicarbonate also depresses water, sodium and bicarbonate reabsorption. These results suggest that chloride administration is not necessary to diminish bicarbonate reabsorption and that tubular reabsorption of bicarbonate depends in part on the state of effective extracellular volume.