The purpose of this project is to develop a system to study the mechanism of action of thyroid hormone at a molecular level, utilizing current techniques of molecular biology. A cDNA library has been prepared from the livers of rats treated with pharmacologic doses of T3, yielding about 25,000 recombinant clones. Some 100 species in this library appear to be responsive to T3, either increasing or decreasing in abundance in the livers of T3 treated animals. Of these clones, 16 have been selected for study in greater depth. These studies include quantitative determination of the abundance of corresponding RNAs in livers of treated and untreated animals; determination of nucleic acid sequences; characterization of cDNA sequences as protein encoding; determination of the subset of corresponding gene products under the transcriptional control of T3.