It was shown by Rees & Rowland (1961) that rat-liver nuclei isolated in 025 M-sucrose incorporated amino acids into protein, and nucleotides into RNA. Although the incorporation was inhibited to varying degrees by anoxia and by several inhibitors of oxidative phosphorylation, it was not affected by detergents, by freezing and thawing the nuclei, or by disruption of the nuclei by ultrasonic vibration (Rees, Ross & Rowland, 1961). We have now studied isolated nucleoli and other subnuclear fractions in order to investigate the active site of synthesis of proteins and nucleic acids within the nucleus. In addition the enzymic and chemical composition of the various fractions has been examined. Part of this work has been published in a preliminary form (Rowland, Rees & Varcoe, 1962). MATERIALS AND METHODS Animals. Male albinorats,weighing200-250g., were used. Reagent8. These were as described by Rees & Rowland (1961). In addition [1-34C]valine and [1-14C]leucine were obtained from The Radiochemical Centre, Amersham, Bucks. Nuclear and8ubnuclearpreparation&. Nuclei were isolated from rat liver in 0*25M-sucrose as described by Rees & Rowland (1961). Scheme 1 shows the method used for isolating nucleoli and other subnuclear fractions. Batches of isolated nuclei, suspended in 15-20 ml. of 0 25m-sucrose, were subjected to ultrasonic vibration (20 keyc./sec.) at 20 in a MSE ultrasonic disintegrator (60w) with a titanium probe of 1 cm. diam. until all nuclei were disrupted (usually 7 min.). The resulting suspension was centrifuged for 30 sec. at 2500g to remove probe debris and coagulated protein. The nucleoli I Sediment Recentrifuged twice in water (see text) Nucleoli (fraction A) Disrupted nuclei Centrifuged at 412 x 104g,^min.