The regulation of gene expression by hormones can be implemented at different levels: mRNA transcription, processing, transport from the nucleus into the cytoplasm, and translation. In the authors' earlier papers it was shown that the thyroxin hormone-receptor complex (HRC) increased chromatin matrix activity and induced new sites of transcription initiation. The present work was concerned with a study of the role of the thyroxin HRC in the regulation of the mechanism of 3H-RNA transport from the nucleus into the cytoplasm. An idea of the involvement of the HRC cytoplasmic thyroxin in RNA transport resulted from the detection in 30-40 S of RNP particles from rat liver nuclear proteins similar to the thyroxin cytoplasmic receptor by the antigen determinants. These data combined with others on the stimulation of 3H-RNA discharge from isolated nuclei under the influence of the thyroxin cytoplasmic HRC led to a conclusion that the thyroxin cytoplasmic HRC was involved in the regulation of gene expression by the thyroid hormones not only at the stage of transcription but also at the post-transcription stage, namely at the stage of RNA nuclear-cytoplasmic transport.