Specific radioactivity of RNA from rat liver tissue, labelled with 14C-orotic acid and fractionated using thermic phenol method, was altered dissimilarly. In single stimulation of hypothalamus the specific radioactivity of nuclear RNA was increased in RNA-40 degrees fraction than in RNA-55 degrees and RNA-63 degrees ones. A decrease in synthesis of nuclear RNA fractions followed the phase of activation in repeated stimulation of hypothalamus. The distinct increase in specific radioactivity was observed in cytoplasmic RNA (RNA-4 degrees) both in single and repeated stimulation of hypothalamus. The stimulating effect of the prolonged hypothalamus irritation on synthesis of the RNA fractions and on activity of glucose-6-phosphatase and fructose-1,6-diphosphatase disappeared more rapidly than this effect in long-term administration of hydrocortisone. The data obtained suggest that sensitivity of tissue-targets to prolonged hormonal stimulation is distinctly higher than the ability of hypothalamus to activate the synthesis of RNA and enzymatic proteins.