Parathyroid hormone (PTH) inhibits renal cortical carbonic anhydrase. Based on this in vitro study, PTH depresses proximal reabsorption of phosphate and bicarbonate reabsorption in vivo by inhibiting carbonic anhydrase. To test this bicarbonate and phosphate excretion were measured in 4 groups of dogs. Group I received PTH for 2 h; group II received acetazolamide for 2 h; group III received PTH for 2 h and acetazolamide in the 2nd h; and in group IV, acetazolamide was given for 2 h with PTH added in the 2nd h. Acetazolamide administration resulted in maximal phosphate and bicarbonate excretion in the 1st h. PTH resulted in maximal bicarbonate excretion in the 1st h and maximal phosphate excretion in the 2nd h. Addition of acetazolamide to animals receiving PTH or addition of PTH to animals receiving acetazolamide resulted in additional increases in bicarbonate and phosphate excretion. PTH induces bicarbonate and phosphate excretion regardless of whether carbonic anhydrase is intact or nearly 100% inhibited by acetazolamide. These data do not support the hypothesis that PTH inhibits bicarbonate and phosphate reabsorption by inhibiting carbonic anhydrase.