The reduction of uridine di- and triphosphate is being studied in normal and tumor tissues. The enzyme involved which appears to be different from cytidine diposphate reductase is being purified and its properties will be studied in detail. The distribution and activity of the enzyme in various normal and tumor tissues will be determined and an effort will be made to see whether uridine reduction could serve as an alternate pathway for the production of thymidine nucleotides. Various related enzymes such as dUTP pyrophosphatase are being studied particularly with respect to mechanisms which control their synthesis and activity.