A rapid detergent method for the isolation of nuclei from cat brain cortex is described. It involves the homogenization of the tissue in buffered 0.34 M sucrose with the addition of the non-ionic detergent Cemulsol NPT 12 and the subsequent low speed centrifugal sieving of the nuclei through two layers of sucrose (0.68 M and 1.0 M ). The final purification is achieved by high speed centrifugation (40,000 g ) of the nuclear suspension layered over 1.8 M sucrose. Observations by light microscopy indicate that highly purified and well preserved nuclei are obtained from neurons and glial cells. Electron microscopy reveals some microsomal contaminants adhering to the nuclear membrane. According to an analysis of the nuclear size distribution, a considerable loss of smaller nuclei (10 to 20µ 2 ), mainly from glial cells, occurs during the purification procedure. The action of different detergents is compared, the best results being obtained with Cemulsol NPT 12 or Triton X-100. Chemical analyses of the purified nuclear fraction give the following content expressed in picograms per nucleus: DNA, 6.54; RNA, 2.94; cholesterol, 1.50; and protein, 97.5. The sucrose density gradient centrifugation of nuclei isolated from cat brain cortex shows that their density is equal to or higher than that of 2.2 M sucrose and is thus similar to the density of nuclei from other tissues. The observation of a varying influence of different suspending media on the density of brain cell nuclei resolves the conflicting data in the literature on the density of these nuclei.