The complexity of the interrelationships of the cyclases, phosphodiesterases, protein kinases, and phosphoprotein phosphatases with macromolecular factors, magnesium, manganese, and calcium ions, and levels of ATP and GTP and, in addition, the existence of multiple enzyme forms with differing properties even within one cell make it difficult or impossible to extrapolate results obtained with homogenates to the functional operation of cyclic nucleotide systems in intact cells. Studies on the accumulation of cyclic nucleotides in brain slices, ganglia, and cultured cells under various conditions can, however, provide pertinent information. The extent of accumulation of cyclic AMP in intact cells will be influenced by a number of factors: (1) the basal levels of adenylate cyclases and the availability of ATP as a substrate; (2) the degree of activation of this enzyme by biogenic amines, adenosine, and other compounds: (3) the levels and properties of cyclic nucleotide phosphodiesterases; and (4) the levels of cyclic AMP-dependent protein kinases with binding sites that presumably sequester cyclic AMP. The subcellular distribution and availability of ions such as calcium and magnesium with their abilities to inhibit or activate these systems will be important in determining the extent of accumulation of cyclic AMP. Similar factors would be involved in the function of cyclic GMP-generating systems of intact cells.