The chromatin extracted from rat hepatocytes of different ages has been shown to contain a phospholipid fraction representing 0.47-0.59 per cent of total chromatin in newborn animals and 0.22 per cent in 45-day-old animals. No such age-related differences are observed in the nuclei. The phospholipid composition of the nuclei at different ages shows a higher level of sphingomyelin and a lower level of phosphatidylserine in newborn than in adult animals. Chromatin phospholipids have a completely different composition from that of nuclei with respect to age, particularly in newborn rats, where there is a decrease in phosphatidylcholine and an increase in phosphatidylserine.