THE IN VIVO TECHNIC of studying the inactivation of certain estrogens and androgens has disclosed that when estrone (1) and the semisynthetic compounds, testosterone propionate (1) and methyl testosterone (2,), are implanted as a pellet in the spleen, they are unable to exert their specific action on the castrate animal of the appropriate sex. The site of inactivation has been considered to be the liver. Additional evidence that the liver plays an important role in this process has been brought out by numerous observers (3, 4). Both Zondek (5) and Heller (6) found that estradiol is inactivated when incubated with liver slices or minced liver. The present experiment was designed to determine if estradiol and estradiol benzpate are inactivated by the liver in vivo. Compressed pellets of estradiol1 and estradiol benzoate1 were prepared in a spedaily designed mold, under a pressure of 6,500 lb. per sq. inch (7).