The correlation between hydration of Nucleic Acid Bases (NABs) and their conformational flexibility was analyzed based on the results of Car-Parrinello Molecular Dynamics (CPMD) simulations of NABs in bulk water environment. Correlations with quantum chemical results were drawn whenever it was possible. Statistical analysis confirmed that hydration causes bond length alteration in NABs and formation of zwitter-ionic resonance structures. In contrast to the gas phase, bulk hydration results in restricted mobility of amino group and increase in population of its planar-like conformations. At the same time, rings of all NABs become almost equally flexible in the dynamic aqueous environment. Therefore, each NAB possesses a non-planar effective conformation of pyrimidine ring despite the fact that planar geometry corresponds to minimum on the potential energy surface.