Ab initio molecular orbital calculations on monocyclic pentaoxyphosphorane molecules as model states in cyclic AMP action are carried out. Five-membered-ring systems are included for comparison. Minimum energy geometries in trigonal bipyramidal configurations that position the rings in axial-equatorial (a-e) sites are compared with those for diequatorial (e-e) ring locations. The models are made increasingly more complex approaching proposed intermediates in enzymatic hydrolysis of cAMP. These contain ribose rings trans-fused to the saturated six-membered ring of the pentacoordinated phosphorus state. All calculations show the e-e ring location to be higher in energy than the a-e ring arrangement. The computed energies and P-O bond lengths are compared with VT NMR activation energies for ligand exchange and with X-ray structural studies, respectively, on pentaoxyphosphoranes. The computations strongly support proposals for in-line enzymatic hydrolysis of cAMP with the ring positioned at a-e sites in a boat conformation.