A significant hyperplasia of the thymus was induced in mice, treated with triiodothyronine during the first month of life. Stereological data showed that, in both treated and control mice, mononucleate epithelial cells were four times more numerous in the medulla that in the cortex. After triiodothyronine treatment, their absolute number in both cortex and medulla increased two-fold. The number of thymic epithelial cells could thus be regulated by thyroid hormones. The cortical volume in treated mice was also twice that of controls but medullar volume showed and increase of only fifty percent. Cortical epithelial cells increased at the same rate of the cortex volume by medullary epithelial cells grew more rapidly. In fact the medullary volume enlargement could be be explained mainly by the growth of the epithelium. Medullary lymphocytes did thus not preliferate in the same way as cortical lymphocytes after thyroid hormone administration. The recently described multinucleate epithelial cells were not modified in number and were thus insensitive to thyroid hormones.