Abstract. We measured sodium-proton (Na + /H + ) exchange in lymphocytes and platelets of a 46-yearold woman with the adult Fanconi syndrome before, during, and after treatment with NaHCO 3 . Kappa light chains in her urine and unique but rarely observed crystalline structures confirmed the presence of lightchain nephropathy. Her glomerular nitration rate was only moderately impaired at 72 ml/min. NaHCO 3 at 1, 3, and 5 mmol/kg/day for 5 days increased her serum HCO 3 and pH from 17 to 21 mmol/1 and 7.28 to 7.39 respectively. Plasma renin and aldosterone values were decreased by NaHCO 3 . Na + /H + exchange (ciHi/min) was measured with the fluorescent marker BCECF after acidification of lymphocytes and platelets with sodium propionate at five (10-50mM) doses. Na + /H + exchange was accelerated in this patient compared to normal controls. NaHCO 3 treatment significantly decreased Na + /H + exchange in lymphocytes, but not in platelets. These findings suggest that Na + /H + exchange can be influenced by NaHCO 3 ingestion at doses that only modestly affect systemic pH. Since Na + /H + exchange is involved in stimulus response coupling, cell growth regulation, cell differentiation, and perhaps the progression of nephrosclerosis, these observations may have clinical relevance.