It has been established that activities of RNA-polymerase, DNA-polymerase, DNA-methyltransferase, and sphingomyelinase in the liver nuclei isolated from newborn (2-day-old) rats are by 18, 25, 27, and 610%, respectively, higher than those in mature (6-month-old) rats. It has been also shown that the number of single-stranded stretches in rat liver nuclear DNA increases with age: it is about 2-fold higher in mature rats than in newborn rats. Thus, the postnatal period of ontogenesis is accompanied by both significant changes in DNA structure and decrease in activities of enzymes serving for the most important genetic processes in cells of animals.