Carbonic anhydrase activity was determined in livers of rats treated with estrogen and/or progesterone. When mature female rats were treated with beta-estradiol (100 mug daily) for 3 days followed by progesterone (25 mg daily) for 2 days, there was a 2-fold increase in hepatic carbonic anhydrase activity. Either hormone alone produced no change. With 21 days treatment, however, estrogen but not progesterone caused a 4-fold increase in hepatic carbonic anhydrase. Similar results were obtained when ovariectomized rats were treated with estrogen or progesterone. The male rat contained a mixture of isoenzymes of which only 20 percent of enzyme was sensitive and the remainder refractory to inhibition by sulfonamides. Castration yielded 70 percent of enzyme sensitive, whereas castration plus progesterone yielded enzyme totally sensitive to sulfonamides without any change in total activity of hepatic carbonic anhydrase. Estrogen treatment of male rats yielded not only 100 percent sensitive enzyme but also caused a 2-fold increase in hepatic carbonic anhydrase. It is concluded that estrogen induces hepatic carbonic anhydrase of the sulfonamide-sensitive type in both males and females. This isoenzyme is normally present in female rats, but in males it replaces the sulfonamide-resistant type normally identified.