The multiple-indicator-dilution technique has been applied to a study of the permeability of proximal and distal portions of the nephrons to dissolved carbon dioxide and bicarbonate ion in anesthetized dogs. Under control conditions with mannitol loading, the excreted carbon dioxide appears considerably earlier than the simultaneously injected creatinine whether dissolved CO2 or bicarbonate ion is injected. After inhibition of carbonic anhydrase by acetazolamide, there is relatively little effect on the excretion pattern of dissolved CO2. However, the excretion pattern of bicarbonate ion becomes nearly parallel to that of creatinine: the early peak disappears. On the basis of these results, it is concluded that the distal portions of the nephrons are permeable to dissolved CO2 but impermeable to bicarbonate ion and that, under control conditions, carbonic anhydrase serves to establish a catalytically mediated diffusion exchange for the transfer of CO2 derived from bicarbonate ion. Similar conclusions may apply to the proximal portions of the nephrons. On taking into account other data, it appears that the collecting ducts are impermeable to dissolved carbon dioxide. Carbon dioxide produced by decarboxylation of pyruvate has excretion patterns similar to those obtained for dissolved carbon dioxide. It is concluded that the decarboxylation product is dissolved CO2 and not bicarbonate ion.