NINE FIOTJRPSThe importance of nucleoproteins to processes of growth and development has been assumed and we have set out to learn something of their precursors with two main goals in mind; first, to find out what compounds are involved in the biosynthesis of nucleoproteins, and second, to investigate structural analogs of those compounds in a search for antimetabolites which could interfere preferentially with rapidly growing tissues. Of necessity most of our attention has so far been focused on the first point.The knowledge, furnished by Plentl and Schoenheimer ( '44), that guanine, uracil, and thymine were not utilized by the rat f o r nucleic acid production was the starting point for these investigations. The purine, adenine, was investigated because of its ubiquitous occurrence not only in both types of nucleic acids but also in coenzymes and in adenosinetriphosphate (ATP). The convenient synthesis of Baddiley, Lythgoe, and Todd ('43) was first used to incorporate N1; into the 1-and 3-positions of adenine (Brown, Roll, Plentl, and Cavalieri, '48).This adenine was administered to rats and it WRS not onlT incorporated as adenine but its isotope was also incorporated into polynucleotide guanine ( fig. 1). Degradation of the isolated guanine showed that the isotope was still in the 1and