Publisher Summary The chapter discusses the data concerning the distribution of free nucleotides in animal tissues and to indicate the importance of such information in the study of the regulation of metabolic processes. The mode of action of free nucleotides displays another aspect of their polyvalent character. Sometimes, they act as coenzymes more or less highly bound to the enzyme and are transformed and reconstituted in one step, as with nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and flavin adenine dinucleotide (FAD); sometimes, the regeneration of the acid anhydride, called coenzyme, necessitates a whole cycle of reactions, as with the uridine diphosphate (UDP), guanosine diphosphate (GDP), and cytidine diphosphate (CDP) derivatives. In some cases, they are simply the substrates of the enzymatic action, as in the biosynthesis of nucleic acids, from ribo- and deoxyribonucleoside triphosphates. The existence of certain cellular compartments, such as the nucleus, where the nucleotide pool is small and the synthesis of certain compounds as NAD and RNA intense show a possible competition for nucleotides, the synthesis of which might be a limiting factor. This shows the importance of exploring free nucleotide pools in tissues and the manner of their regulation.