To test the effect of an added amount of thyroid product on the histological structure of the mouse suprarenal, thyroxin was injected daily into mice in controlled groups, the [female] [female] being about 70 days and the [male][male] about 32 days of age. Normally the reticular zone of the mouse suprarenal begins to degenerate in the first 1/2 of the reproductive period of the [female], but entirely disappears by 40 days of age in the [male]. In the [female] 9[female] used in this study, injections for 28 days caused a marked hyperplasia of the right gland as compared with the left, which had been previously removed. Though all zones had increased, the reticular zone was doubled in size. The normal progress of degeneration had been held almost in abeyance during the period of treatment. In the [male] [male] with the left suprarenals removed, both hypertrophy and hyperplasia took place in the remaining gland, while in those with both glands intact the cortical tissue increased to twice that of the control animals. A histological structure similar to that of the young growing animal was produced in the [male] gland. In those in which the normal disappearance of the reticular zone had been completed before the injections entire regeneration of this zone took place. A marked hyperemia and enlargement of the individual cells showed in the medulla. It is suggested that these glandular changes occur in response to the metabolic disturbances caused by the thyroxin and that they are coincident with the establishment of the endocrine balance of the organism on a new level.