A method is presented for the rapid separation of cell nuclei from the rabbit cerebral cortex into two populations. The first of these consists largely of nuclei with the morphological characteristics of neuronal nuclei, the second almost entirely of nuclei with the morphological characteristics of glial cell nuclei. From studies based upon sensitivity to the toxin α-amanitin, the ratio of incorporation of different bases, ionic requirements and differential sensitivity to actinomycin D, it is concluded that under both the classical low and high salt conditions described by other workers, two enzymes are active in RNA synthesis. The presence of a third enzyme of low activity cannot be excluded. No qualitative difference in the number of enzymes involved in RNA synthesis in neuronal and glial cell nuclei has been found, but there are quantitative differences in activity between the two nuclear populations.