SummaryUsing the stop flow technic in NaHCO3 loaded dogs under osmotic diuresis we find that bicarbonate and chloride are reabsorbed avidly in a far distal area coincident with sodium. Compared to values in free tlowing osmotic diuretic urine, concentration of bicarbonate developed in proximal tubules during stop flow remains low at the same point where chloride is relatively elevated. Reabsorption of proximal HCO3 alone does not account for Cl elevation. Cl secretion into and removal of other anions from the proximal fluid is considered.