The general concept concerning mechanism of steroid hormone action includes several steps at the molecular level about which very little is known. ·For several steroid responsive tissues it has been shown, that the steroid molecule after entering its target cell, becomes immediately bound by a "cytoplasmic receptor" which transfers the steroid into the nucleus. This translocation step is temperature dependent. Simultaneously the receptor part of the complex is transformed into a smaller subunit, the "nuclear receptor". In the nucleus the steroid receptor complex becomes attached to an acidic non histone protein, the "nuclear acceptor" site, and provokes an increase in RNA polymerase activity resulting in new RNA synthesis followed by increased protein synthesis.