Hydration of the fluoroquinolone antibiotic norfloxacin has been examined in B3LYP/6-31+G* calculations. The neutral and zwitterionic forms and their one- and two-water complexes have been optimized both as isolated molecules and within a solvation field simulated by the polarizable continuum model. The explicitly included water molecules are found to affect solvation energies of both neutrals and zwitterions, but overall their inclusion does not improve the average solvated neutral-zwitterion energy separation. The calculated separations of 3-5 kcal/mol are consistent with experimental observations that the two forms coexist in solution under physiological conditions. (c) 2006 Elsevier B.V. All rights reserved.