The administration of cortisone to rapidly growing rats results in a prompt and virtually complete inhibition of liver DNA synthesis. This occurs early and at a time when there is a concomitant stimulation of synthesis of liver RNA, protein and glycogen. Inhibition of liver DNA synthesis is sustained for as long as hormone is administered and rapidly returns to normal upon cessation of hormone treatment. Inhibition can be observed in animals receiving as little as 0.3 mg of cortisone/100 g/day, and the doses required for inhibition of liver DNA synthesis and for suppression of somatic growth are similar. It is of particular interest that inhibition of liver DNA synthesis can be observed with doses of hormone close to the “physiological” range and that even very large doses of hormone fail to result in appreciable degradation of liver DNA. (Endocrinology88: 1471, 1971)