In metabolic acidosis, the capacity of the proximal tubule for bicarbonate absorption is enhanced, whereas NaCl reabsorption is inhibited. Recent evidence indicates that transcellular NaCl absorption in the proximal tubule is mediated by apical membrane Cl/formate exchange and Cl/oxalate exchange, in parallel with recycling of these organic anions. We evaluated whether the effect of metabolic acidosis to inhibit NaCl reabsorption in the proximal tubule is due at least in part to inhibition of organic anion-dependent NaCl transport in this nephron segment. Absorption rates of bicarbonate (JHCO3), chloride (JCl), and fluid (Jv) were measured in rat proximal tubule segments microperfused in situ. We confirmed that metabolic acidosis stimulates JHCO3 in tubules microperfused with 25 mM HCO3, pH 7.4. For measurements of JCl, tubules were microperfused with a low-bicarbonate (5 mM), high-chloride solution, simulating conditions in the late proximal tubule. Under these conditions, baseline JCl and Jv measured in the absence of formate and oxalate were not significantly different between control and acidotic rats. However, whereas addition of 50 ¿M formate or 1 ¿M oxalate to luminal and capillary perfusates markedly stimulated JCl and Jv in control rats, formate and oxalate failed to stimulate JCl and Jv in acidotic rats. We conclude that metabolic acidosis markedly downregulates organic anion-stimulated NaCl absorption, thereby allowing differential regulation of proximal tubule NaHCO3 and NaCl transport.