Iodine ions exhibited the thyroxin-like effect on incorporation of 1-14C-leucine into proteins of isolated mitochondria and microsomes of thyroidectomized rats in vitro. Thyroxin, triiodothyronine (T3) and ICl increased the incorporation of 1-14C-leucine into proteins of isolated mitochondria of thyroidectomized rats, but did not affect the protein synthesis in microsomes in vitro. Rifampycin and olivomycin abolished completely the stimulating effect of T3 and ICl on incorporation of the label into mitochondrial proteins. The thyroid hormones and iodine ions stimulated protein synthesis in vitro in liver microsomes of thyroidectomized animals only after preincubation with mitochondria or nuclei. In these conditions preincubation with mitochondria elevated the rate of 1-14C-leucine incorporation into microsomal proteins 2--2.5-fold. In similar experiments with nuclei--4--4.8-fold stimulation was detected. Thyroid hormones and iodine ions stimulated synthesis of specific factors in mitochondria (MBS) and in nuclei (NBS) of thyroidectomized rat liver tissue, which increased the protein synthesis in isolated microsomes in vitro. Synthesis of MBS- and NBS-factor required the presence of all the four ribosetriphosphates (ATP, GTP, UTP, CTP) and was inhibited completely by olivomycin; rifampycin blocked only the MBS factor synthesis. NBS- and MBS-factors appear to be RNA (mRNA), synthesized in nuclei and mitochondria, which are transported into the incubation media and translated by ribosomes.