Pn entering the cell, steroid hormones are bound to specific cytoplasmic receptors. The hormone-receptor complexes are then translocated to the nucleus, in an "activated" form, whereupon they are bound to the target cell genome. The target cell responds by increased RNA synthesis with the transcription of specific mRNAs. The mRNAs are exported to the cytoplasm where protein synthesis takes place. Recent advances in steroid hormone action involve the purification of specific steroid hormone receptors and the preparation of specific antisera, the elucidation of the subunit structure of the progesterone receptor, the purification of a hormone-inducible mRNA and the synthesis of its DNA complement, the purification and amplification of the synthetic and natural gene for this RNA and the identification of intragenic spacers, and most recently the identification of specific precursors to the hormone-induced mRNA. We discuss here the medical significance of some of these advances.