This study was undertaken to observe the effect of xanthine such as caffeine and aminophylline on the activity of carbonic anhydrase in the kidney and stomach of the mouse. Carbonic anhydrase activities were measured by Philpot & Philpot method (1936). The results of this experiment were as follows: 1. The activity of carbonic anhydrase in the kidney of the mouse was silightly inhibited by the administration of caffeine (0.1 mg/gm, B.W.) or aminophylline (0.08 mg/gm, B.W.). The inhibition was more pronounced by the administration of aminophylline than that of caffeine. 2. In the stomach, there was no significant change in the activity of the carbonic anhydrase after the administration of either caffeine or aminophylline.