The effect of thyroid hormone on acetylation, phosphorylation and ADP ribosylation of rat liver nucleoproteins was studied by incubating intact nuclei with labeled precursors. Acetylation, which occurred in histones and low molecular weight proteins (less than 30,000), was depressed in nuclei from thyroidectomized animals. The administration of L-3,5,3'-triiodothyronine (T3) increased acetate incorporation to 50% over control levels. Incorporation of labeled phosphate from ATP into most proteins was decreased in nuclei from thyroidectomized animals and increased by the administration of T3. The greatest increase produced by T3 (to 140% of control values) was seen in proteins of molecular weight greater than 68,000. Nuclei from thyroidectomized animals incorporated less ADP ribose in most proteins. Both high molecular weight proteins (greater than 68,000) and low molecular weight proteins (less than 30,000) showed a further decrease in ADP ribose incorporation in nuclei from thyroidectomized rats given T3. However, a few proteins of the middle molecular weight class showed increased ADP ribose incorporation subsequent to the injection of T3. It is suggested that a generalized increase in protein synthetic rates previously noted to be caused by T3 is accompanied by increased acetylation and phosphorylation of histones and other proteins. These changes could accelerate transcription of already active genes.