Recent findings have led to a greater understanding of thyroid hormone action. The nuclear receptor for triiodothyronine is an integral component of a larger chromatin fragment. A stereospecific energy-dependent transport system appears responsible for translocation of triiodothyronine from cytosol to nucleus. In the liver, a multiplicative interaction between a signal from the triiodothyronine-nuclear receptor complex and a signal generated from carbohydrate metabolism results in the induction of specific mRNAs. Two-dimensional mRNA activity profiles suggest that approximately 8% of the visible mRNA sequences are differentially affected by alterations in thyroid states. Almost 30% to 40% of these changes are mediated by an increase in pituitary growth hormone induced by triiodothyronine. Sequential analyses of mRNA activity profiles have identified an mRNA sequence (mRNAs14) coding for a protein (S14) with Mr 17 010 and pI 4.9 which responds to triiodothyronine with a lag time of less than 20 minutes. The coordinate regulation of mRNAs14 by carbohydrate and triiodothyronine and its presence in lipogenic tissues (fat, liver, lactating mammary tissue) suggests that S14 is involved in some aspect of fatty acid synthesis degradation or storage.