The incorporation of 14C-leucine in protein, as well as in aminoacyl adenylate and aminoacyl-tRNA (AA-tRNA) from the liver of mature female rats (controls, ovariectomized and treated after the ovariectomy with daily doses of 20 micrograms 17, beta-estradiol-dipropionate/100 g body weight for 11 days), was investigated under conditions of a cell-free protein-synthesizing system containing isolated microsomes and cytosol. The ovariectomy causes reduced incorporation of the label in protein and in AA-tRNA, while hormonal treatment compensates this decrease either partially or completely. Data are presented which suggest that the hormone probably increases the supply of macroergic components and free amino acids to the hepatocytes, and that it also increases the sensitivity to cycloheximide and tetracycline. It is assumed that estradiol participates in the regulation of the protein synthesis in the hepatocytes at the level of AA-tRNA synthesis.