To date, relative cellular levels of cGMP and cGMP-binding proteins have not been considered important in the regulation of smooth muscle or any other tissue. In rabbit penile corpus cavernosum, intracellular cGMP was determined to be 18 ± 4 nm, whereas the cGMP-binding sites of types Iα and Iβ cGMP-dependent protein kinase (PKG) and cGMP-binding cGMP-specific phosphodiesterase (PDE5) were 58 ± 14 nm and 188 ± 6 nm, respectively, as estimated by two different methods for each protein. Thus, total cGMP-binding sites (246 nm) greatly exceed total cGMP. Given this excess of cGMP-binding sites and the high affinities of PKG and PDE5 for cGMP, it is likely that a large portion of intracellular cGMP is associated with these proteins, which could provide a dynamic reservoir for cGMP. Phosphorylation of PDE5 by PKG is known to increase the affinity of PDE5 allosteric sites for cGMP, suggesting the potential for regulation of a reservoir of cGMP bound to this protein. Enhanced binding of cGMP by phosphorylated PDE5 could reduce the amount of cGMP available for activation of PKG, contributing to feedback inhibition of smooth muscle relaxation or other processes. This introduces a new concept for cyclic nucleotide signaling.