1. Methods of estimation of ribose nucleic acid are discussed and a reliable method for the estimation of that substance in isolated cell nuclei described. 2. An inhibitory effect of deoxyribose nucleic acid on the production of furfural by acid hydrolysis of ribose nucleic acid is discussed. 3. Methods are given for the isolation of various types of cell nuclei. 4. Results are Quoted and discussed for the ribose nucleic acid content of cell nuclei isolated from normal and regenerating tissues. 5. .the histochemistry of the nucleolus is discussed in relation to the above results and evidence for the presence of ribose nucleic acid in the nucleolus given. b. A criticism of Brachet's histochemical test for ribose nucleic acid is given,evidence being produced to suggest that the current interpretation of the above test is faulty. 7. The occurrence of ribose nucleic acid in the nucleolus is postulated.