This study was carried out to observe the effect of testosterone on carbonic anhydrase inhibiting action of acetazolamide. Carbonic anhydrase activities in the kidneys of mice were measured by Philpot and Philpot method(1936) at 30, 90 and 150 minutes after intravenous administration of saline(0.5 ml/10 g) or acetazolamide (0.25 mg/10 g) in mice pretreated with testosterone (0.1 mg/10 g). The changes in volume and pH of urine as well as those in urinary electrolytes, such as Na⁺, K⁺ and Cl^- were measured at 15 minutes interval for 150 minutes in the rabbit pretreated with double administrations of testosterone(10 mg/kg), 1 hour and 18 hours, prior to the administration of acetazolamide (10 mg/kg). The results were as follows: 1. Carbonic anhydrase activities in the kidneys of mice of testosterone-pretreated groups were significantly higher than those of acetazolamide-treated group at 30 minutes. No significant changes of carbonic anhydrase activities were observed in testosterone-pretreated groups compared with saline-treated groups. 2. Combined administrations of acetazolamide and testosterone exhibited higher carbonic anhydrase activity than those group of acetazolamide alone in the kidney of mice through observed period of 150 minutes. 3. There were no significant changes in the excretion rate of urine and urinary electrolytes in the group of rabbits with testosterone administerone alone. Urine volume as well as Na⁺ and Cl^- excretion rates in the combined treated group of acetazolamide and testosterone were significantly lower than that of acetazolamide group throughout experimental period except 15 minutes after drug administration at the time transient increase was shown. 4. Generally lower K⁺ excretion rate was observed in the combined treated group of acetazolamide and testosterone compared with the single acetazolamide-treated group and the testosterone-pretreated group shows lowest excretion rate of potassium.