Acute hyponatremia was induced in rats by subcutaneous pitressin and intraperitoneal injection of 17.5 ml of 2.5% dextrose three times over 2 days. Brain pH was measured by use of 31P-nuclear magnetic resonance (31P-NMR). Two-hour infusion of a 500 meq sodium solution raised plasma sodium concentration (PNa) from 124.6 to 147 meq/l. Brain pH rose 0.1 pH units in the absence of changes in phosphocreatine or ATP. By contrast, increasing PNa in normal rats from 141 to 158 meq/l did not affect brain pH. Brain buffering was examined by 58-min exposure to 20% CO2 followed by a 38-min recovery. Intrinsic brain buffering (Bi) in hyponatremic and normal rats following 15 min of CO2 was similar, 49.1 and 45.5, respectively. After the next 40 min of CO2 exposure Bi was unchanged in hyponatremic rats at 45.8 but increased in controls to 102.5. In recovery Bi was higher initially in controls and rose significantly only in the controls. These results are consistent with the hypothesis that acute hyponatremia impairs the function of the Na+-H+ exchanger in brain.