1. In the rat cerebral cortex net DNA synthesis ceases when the animal has reached about 25g. body weight (18 days of age). There is then little further change in the DNA content per cortex. 2. Nuclear and transfer RNA follow a similar pattern to DNA. 3. Microsomal and ribosomal RNA content increases up to 25g. body weight but then declines. The decrease in ribosomal and microsomal RNA content is associated with a change in RNA base composition. 4. Incorporation of [(14)C]orotic acid into nuclear RNA proceeds at a similar rate in 4-day-old and adult animals. However, there is a lag period of about 60min. in the young animals during which incorporation into the ribosome fractions proceeds slowly. In the adult animals the lag period is not seen.