The admn. of thyroxine to adult male mice was followed by an increase in the organ wts. of the spleen, lymph nodes, kidneys, and adrenal glands without significant effect on the body wt. Similar results were obtained in thyroxine-treated adrenalectomized mice. These effects were independent of change in either rate of growth or in the level of adrenal cortical activity. Following treatment with thyroxine, thymus wts. were decreased in intact animals and somewhat increased in adrenalectomized animals. This suggests that the involution of the thymus following treatment may be due to a concomitant increase in adrenal cortical activity.