The rate of both the synthesis of liver and plasma proteins and RNA-1 and RNA-2 polymerase activities in liver were studied in rats of various ages at experimental hypothyroidism. There has been marked more significant decrease in plasma protein synthesis with age. Both the rate of liver and plasma protein synthesis have been shown to be reduced at experimental hypothyroidism but synthesis of plasma proteins was inhibited to a greater extent. Considerable changes were observed neither in liver and plasma protein synthesis nor in the balance between these two groups of the protein synthesis in old rats. RNA-1 and RNA-2 polymerase activities decreased at hypothyroidism. At all ages the activity of bound enzymes decreased to a larger extent as compared to free forms. The activity of RNA-polymerase 2 was more inhibited than that of RNA-polymerase 1. Reducing protein- and RNA-synthetic processes in the liver with age correlated with the peculiarities of the carbohydrate metabolism: in those young animals with an impaired glucose tolerance the inhibitory effect of hypothyroidism on the intensity of protein and RNA synthesis was more potent as compared to old animals.