AbstractThe activity of CTP synthetase (UTP: ammonia ligase (ADP, EC. 6.3.4.2.)) was determined in rat cerebral hemispheres and cerebellum during postnatal development. It was found that enzyme activity, when expressed per unit of protein or wet tissue weight, in both parts of the brain decreased with age, the diminution proceeding more rapidly in the cerebellum than in the hemispheres. When CTP synthetase was expressed on the basis of DNA an activity peak in the hemispheres with a maximum on the 5th day after birth was observed. The enzyme content per cell was several times lower in the cerebellum than in the hemispheres at each age studied, with the exception of 1‐day‐old animals, in which both tissues displayed intensity of RNA synthesis in the developing brain. The conclusion was reached that the changes of CTP synthetase activity were closely related to the RNA metabolism. Therefore the enzyme under study may be one of the factors involved in cell control of RNA synthe