The data discussed herein demonstrate the great variation in target-tissue response that can occur after administration of steroid hormones. The female sex steroids can exert regulatory effects on the synthesis, activity, and possibly even the degradation of tissue enzymes and structural proteins. Each response, nevertheless, appears to be dependent on the synthesis of nuclear RNA. In many instances, the steroid actually promotes a qualitative change in the base composition and sequence of the RNA synthesized by the target cell, implying a specific effect on gene transcription. Most important is our direct quantitative evidence that sex steroids cause a net increase in the intracellular amounts of specific mRNA molecules in target tissues. It thus appears that we are discovering a pattern of steroid hormone action which includes (Fig. 1): (i) uptake of the hormone by the target cell and binding to a specific cytoplasmic receptor protein; (ii) transport of the steroid-receptor complex to the nucleus; (iii) binding of this "active" complex to specific "acceptor" sites on the genome (chromatin DNA and acidic protein); (iv) activation of the transcriptional apparatus resulting in the appearance of new RNA species which includes specific mRNA's; (v) transport of the hormone-induced RNA to the cytoplasm resulting in synthesis of new proteins on cytoplasmic ribosomes; and (vi) the occurrence of the specific steroid-mediated "functional response" characteristic of that particular target tissue. To elucidate fully the mechanism of steroid hormone action we must study the biochemistry of the process by which information held by the steroid hormone-receptor complex is transferred to the nuclear transcription apparatus. If our assumptions are correct, we should ultimately be able to discover how this hormone-receptor complex exerts a specific regulatory effect on nuclear RNA metabolism. Such regulation might be achieved (i) by direct effects on chromatin template leading to increased gene transcription and thus RNA synthesis; (ii) by activation of the polymerase complex itself; (iii) by inhibition of RNA breakdown; or (iv) by intranuclear processing of large precursor molecules so that smaller biologically active sequences are produced, and (v) by transport of RNA from the nucleus to the cytoplasmic sites of cellular protein synthesis.