Abstract Methods are described for the isolation and degradation of deoxyribose from deoxyribonucleic acid. In regenerating liver, ascites tumor, and HeLa cells the pattern of labeling in deoxyribose formed from specifically labeled glucose or ribose is similar to that found in ribose. Differences observed in the relative labeling of C-1 and C-2 of ribose and deoxyribose can be interpreted in terms of two pathways of ribose synthesis. However, in experiments with C-1 labeled glucose, C-5 of deoxyribose was heavily labeled, indicating a possible triose phosphate precursor. It is unlikely that acetaldehyde plays a significant role in the in vivo formation of deoxyribose.