Ab initio (HF, MP2, and CCSD(T)) and density functional theory computed results on the equilibrium geometries, relative stabilities, strain energies, and vibrational spectra of the nine possible valence isomers of pyridine are reported. Although some aza-benzvalenes (V1N and V3N) lie lower in energy than Dewar pyridines (D1N and D2N), the strain energies for the latter are lower. Relative stabilities of the valence isomers, thermodynamic stability. and skeletal rigidity are comparable to those of benzene valence isomers.