Nucleic acid analyses were made on the various areas of the rat brain using the ultraviolet absorption method. It was found necessary to use fresh tissues to obtain correct ribonucleic acid concentrations since analyses in some areas (cerebellum and hypothalamus) were lower when the tissues had been kept frozen. Enzymatic analysis indicated that the higher concentration of 5'nucleotidase in cerebellum than in cortex might be indirectly responsible for the loss of the ribonucleic acid during the freezing and thawing processes. Deoxyribonucleic acid concentration in the cerebellum is approximately 4-5 times the amount found in the other areas. Ribonucleic acid content of the brain decreased in the order: cerebellum, hypothalamus, cerebral cortex, thalamus, white matter and medulla. Average cell densities, weight/cell, and total cells/area were calculated from the deoxyribonucleic acid concentrations. Close agreement between the calculated cell densities and direct measurements were found.