Effect of protein factors from heart and liver cytoplasm (S-100 fraction) on activity of RNA polymerases I and II was studied in isolated nuclei of heart and liver tissues under conditions of immobilization stress. Activity of the cytosol factors, stimulating ribosomal RNA (rRNA) synthesis, was altered during the post-stressory period. Immediately after immobilization the activity of the cytosol was decreased at the catabolic phase. Within a day the activity was distinctly increased at anabolic phase of the stressory reaction. The active post-stressory cytosol was studied in the cell-free system containing nuclei from heart and liver tissues of animals subjected to stress and of the control animals. The active post-stressory cytosol increased the rate of rRNA synthesis in corresponding nuclei of heart and liver tissue, while synthesis of rRNA was increased 2-fold in nuclei of control animals as compared with heart and liver nuclei, isolated within a day after termination of stress. Activation of the cytoplasmic factors stimulating the transcription rate is concluded to be of importance for regulation of rRNA synthesis under adaptation conditions.