Effect of cytoplasmic fraction S-100 from normal heart and liver tissue as well as from the tissues at the period of compensatory growth, caused by hyperfunction, on the rate of transcription in isolated nuclei was studied. In normal state heart and liver cytoplasm contained factors, accelerating RNA synthesis in nuclei, while activity of RNA-polymerase I was increased most significantly in presence of these factors as compared with RNA-polymerase II. Activity of these cytoplasmic factors, accelerating transcription, was distinctly increased in heart hyperfunction and in the liver tissue remaining after partial hepatectomy. The heart and liver cytoplasmic factors proved to be of protein nature, they did not exhibit any tissue specificity and their activity correlated with the rate of cellular RNA synthesis. Activation of cytoplasmic protein factors, accelerating transcription, appears to be an important mechanism, initiating the RNA synthesis increase in response to elevation of a tissue physiological functions.