1. The synthesis of nucleic acids and the content of purine nucleotides have been studied in selected purine-requiring strains of Escherichia coli including a purB− strain and a purB− guaA− strain. 2. When the exogenous purines can be converted into GTP but not into ATP, RNA is synthesized at the expense of intracellular ATP, ADP and AMP. 3. Net synthesis of RNA as measured by the incorporation of uracil can be correlated with the availability of GTP except when ATP falls to a very low concentration. 4. Nicotinamide nucleotides are not an important reservoir of adenine nucleotides for RNA synthesis.