Mechanisms of the effect of estradiol on the biosynthesis of ceruloplasmin (CP) in rat liver were studied. The radioimmunological tests revealed that estradiol caused an increase of CP concentration in plasma. The about two-fold increase of plasma CP level was accompanied by the proportional increase of the content of CP nascent chains in membrane-bound polysomes of liver. Cell-free translation of free and membrane-bound polysomes revealed that there is a two-fold increase in the relative rate of CP synthesis by membrane-bound polysomes from estradiol-treated rats, while free polysomes did not contribute to CP synthesis. The similar increase of the rate of CP synthesis was found also when RNA preparations from membrane-bound polysomes and from post-polysomal supernatant were translated in a heterologous cell-free system. Studies on the kinetics of hybridization with a specific cDNA probe showed that the increase in the rate of CP synthesis correlates to the increase of the steady-state level of CP mRNA in both membrane-bound polysomes and post-polysomal supernatant. A conclusion is made that the induction of CP by estradiol is related to the accumulation of CP-mRNA in liver cells.