The effect of triiodothyronine (L-T₃) on the protein, RNA and DNA fractions of the thymus nuclei of growth-arrested sulfaguanidine-fed rats has been studied. A single dose of 15 µg of L-T₃ stimulated the incorporation of ¹⁴C-labelled amino acids into total nuclear protein, whole histone and histone fractions. The incorporation of the labelled amino acid into nuclear protein was increased 40 percent over the control 4 hours after the administration of L-T₃. The incorporation of alanine-1-¹⁴C into the total histone fraction reached a maximum 4 hours after the administration of L-T₃ and then decreased 8 and 16 hours after the administration of L-T₃. The incorporation of alanine-1-¹⁴C into histone fraction f₁ reached a maximum at 4 hours after the administration of L-T₃ while the incorporation into fractions f_2a, f_2b and f₃ did not reach a maximum until 8 hours after the administration of L-T₃. Time course studies showed that the protein and RNA moieties of thymus chromatin of rats injected with a single dose of L-T₃ was increased to a maximum at 4 hours. The T_m of chromatin was increased 2° by L-T₃ administration as early as 2 hours. However, the T_m of DNA was not affected. Four hours after the administration of L-T₃ the template efficiency increased 171 percent. The template efficiency of DNA was not affected by L-T₃. These results demonstrate that thyroid hormones modify the chromatin in such a way that the template efficiency is increased. An hypothesis for the mechanism of thyroid hormone action was presented. / Ph. D.