It is proposed to develop the analytical potential of gas chromatography -mass spectrometry for the determination of the in vivo metabolism and distribution of pyrimidine C-nucleoside periodate oxidation products in the urine and tissues of test animals and to survey the urine of human patients with cancer for modified nucleosides. Instrumental techniques to be studied include rapid scanning, multiple ion detection, multiple ion detection and chemical ionization. As a corollary, techniques for the incorporation of stable isotopes into nucleic acid bases, nucleosides, and nucleotides will be developed. These specifically labeled compounds will be used as internal standards in the metabolism and distribution studies.