In the proposed studies we hope to determine the mechanism of thyroid hormone action. There will be an intensive effort to purify, using affinity chromatography and conventional techniques, the intranuclear receptors for thyroid hormone. In addition, the binding of these receptors by DNA, and their localization in chromatin, will be investigated. Finally, properties of the purified receptors in terms of potential biological actions of the receptor will be investigated. The results will also be correlated with thyroid hormone-regulated influences on a specific mRNA, that for growth hormone.