If the protein biosynthesis was inhibited by cycloheximide, an elevated degradation of total mRNA, including mRNA of membrane-bound and free polyribosomes, occurred in the liver cells at early steps of regeneration, within 48 hrs after partial hepatectomy. Simultaneously, distinct increase in activity of alkaline RNAases was observed in cytoplasm as well as of endo-RNAase--in the membrane-bound and free polyribosomes. As shown in experiments with-p-chloromercuribenzoate, the increase in activity of alkaline RNAases in treatment with cycloheximide was due to a decrease in content of short-living proteins, which are inhibitors of RNAases, in cytoplasm of 48 hr regenerating liver cells. At the later period of liver tissue regeneration within 5 days after partial hepatectomy), under conditions of inhibition of protein biosynthesis using cycloheximide, stabilization of mRNA from membrane-bound polyribosomes was observed while the mRNA from free polyribosomes was in a state of degradation. Within this period the endo-RNAase activity was markedly decreased in membrane-bound polyribosomes but the enzymatic activity was increased in free polyribosomes. The mechanisms, controlling the mRNA stability, were apparently changed during the regeneration of rat liver tissue.