Using small-angle x-ray scattering, we have observed the cGMP-induced elongation of an active, cGMP-dependent, monomeric deletion mutant of cGMP-dependent protein kinase (Δ ^1–52 PKG-Iβ). On saturation with cGMP, the radius of gyration of Δ ^1–52 PKG-Iβ increases from 29.4 ± 0.1 Å to 40.1 ± 0.7 Å, and the maximum linear dimension increases from 90 Å ± 10% to 130 Å ± 10%. The elongation is due to a change in the interaction between structured regulatory (R) and catalytic (C) domains. A model of cGMP binding to Δ ^1–52 PKG-Iβ indicates that elongation of Δ ^1–52 PKG-Iβ requires binding of cGMP to the low-affinity binding site of the R domain. A comparison with cAMP-dependent protein kinase suggests that both elongation and activation require cGMP binding to both sites; cGMP binding to the low-affinity site therefore seems to be a necessary, but not sufficient, condition for both elongation and activation of Δ ^1–52 PKG-Iβ. We also predict that there is little or no cooperativity in cGMP binding to the two sites of Δ ^1–52 PKG-Iβ under the conditions used here. Results obtained by using the Δ ^1–52 PKG-Iβ monomer indicate that a previously observed elongation of PKG-Iα is consistent with a pure change in the interaction between the R domain and the C domain, without alteration of the dimerization interaction. This study has revealed important features of molecular mechanisms in the biochemical network describing PKG-Iβ activation by cGMP, yielding new insight into ligand activation of cyclic nucleotide-dependent protein kinases, a class of regulatory proteins that is key to many cellular processes.