Steroids and the Control of Deoxyribonucleic Acid Synthesis in the Uterus

hormone. This possibility has been examined by preparing informofers [nuclear ribonucleoprotein particles containing DNA-like RNA (Samarina el al., 1968)] from isolated uterine nuclei. When immature rats are treated with oestradiol-178 for 1 4 h , and the uterine RNA labelled with uridine during a subsequent incubation in vitro, the synthesis of RNA in informofers is stimulated severalfold. We conclude that, when oestradiol-178 initiates the development of the quiescent uterus, the ‘switching on’ of the ribosome-making machinery is an important early stage of the process. In some little-understood way the fine control of this process is overridden when the uterus is removed and investigated in vitro.