The reabsorption of bicarbonate by the kidneys under normal circumstances appears to be a linear function of the plasma CO, tension (1-3). PreviOtis studies from this laboratory (4) (lemonstratel that when carbonic anhydrase was inhibited by acetazolamide the linear relationship between plasma CO2 tension and HCO,-reabsorption still obtained. It was therefore proposed that the uncatalyzed, as well as the catalyzed, hydration of CO2 was an important source of the H' involved in the reabsorption of HCO-.The present investigations were undertaken in an attempt to characterize more precisely the catalyzed and uncatalyzed reactions. By varying plasma CO2 tension, carbonic anhydrase activity, and filtered HCO3-, three aspects of HCO-reabsorption were examined: 1) the maximal reabsorptive capacity, or the HCO:,-Tm, with and without carbonic anhydrase activity; 2) the relation of HCOJ-excretion to HCO3-Tm with and without carbonic anhydrase activity; 3) the capacity of high plasma CO2 tensions to effect complete HCO3-reabsorption in the absence of carbonic anhydrase.On the basis of these studies it was concluded that HCO;-reabsorption is mediated by two distinct processes. One process has a HCO-Tm which is dependent on plasma pCO2 and independent of carbonic anhydrase activity. The second process is dependent on carbonic anhydrase, independent of plasma pCO2, and necessary for

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