Mass spectrometry will be applied to structural and analytical problems involving naturally modified nucleosides. Structures of new nucleosides from transfer ribonucleic acid will be studied, principally using high resolution mass spectrometry. Quantitative assays of modified nucelosides based on GC/MS will be developed. Systematic studies of the mass spectrometric behavior of nucleoside derivatives will be continued, based on specifically 2H-and 18O-labeled models of uridine.