INTRODUCTION It is believed that in target tissues steroid hormones are bound by cytoplasmic receptors* of high affinity and specificity; that the steroid receptor complex moves to the nucleus where physiological changes are initiated via a modification of genetic expression. The aim of this review is to examine the evidence for the receptor concept, its generality and physiological significance. Aspects which have already been reviewed extensively will be treated summarily. The effects of steroids on nucleic acid and protein synthesis will, for the most part, be ignored. The reader is referred to the papers of Bruchovsky & Wilson (1968a), Neal (1970), Hamilton (1971), and Mueller (1971) for further information. An early step may be the synthesis of a new mRNA species, the translation of which into protein could be a prerequisite of an increase in rRNA synthesis (Knowler & Smellie, 1971; Baulieu, Alberga, Raynaud-Jammet & Wira, 1972). OESTRADIOL RECEPTORS