Ab initio calculations of the vertical transitions, transition dipole moments, and permanent dipole moments of several low-lying valence states of the fluorescent base 2-aminopurine have been obtained in both the gas phase and in water using the configuration interaction singles method in conjunction with the Onsager reaction field model. The results for the electronic state ordering, excited state energy gaps, and transition dipole moment directions are in generally good agreement with those determined experimentally. The fluorescence Stokes＇ shift for the S-0 --> S-1 transition is computed taking into account the contributions from both intramolecular and solvent reorganization processes. The result is in excellent agreement with experiment and suggests that the magnitude of the shift is due primarily to intramolecular relaxation of the S-1 state.