A study was made of the chemical changes produced in isolated cell nuclei by the hydrolytic procedure used in Feulgen''s method. Hydrolysis with 1 [image] HC1 for 10 min. at 60[degree]C causes a loss of 35% of the dry wt. of the nuclei. This loss includes the whole of the histone originally present. It also includes more than 50% of the nucleic acid, which leaves the nuclei in the form of diffusible fragments of this material. The hydrolysis fluid, after separation from the nuclei, gives an intense Feulgen reaction with Schiff''s reagent. The implications of these results are discussed.