2-Aminopurine is a highly fluorescent analogue of adenine that can be incorporated synthetically into DNA with little perturbation of the native double helical structure. The sensitive dependence of the quantum yield of this fluorophore on nucleic acid conformation has made it an invaluable probe of DNA structure, dynamics, and interactions. To assist in the development of models for the molecular interpretation of fluorescence measurements, the electronic structure of 2-amino-9-methylpurine has been calculated in the ground state and the lowest singlet pi pi* and n pi* excited states. These computations employed the complete active space multiconfigurational self-consistent field method (CASSCF), supplemented by multiconfigurational quasidegenerate perturbation theory (MCQDPT). The predicted energies for pi pi* excitation and emission and n pi" excitation are in good agreement with previous experimental values. The permanent molecular dipoles of the ground and pi pi* excited states are similar in magnitude and direction, consistent with experimental observations of weak solvatochromic shifts in pi pi* absorption and emission spectra. However, the permanent dipole of the n pi* state is rotated approximately 60 degrees relative to that of the ground state, implying that the n pi* excitation energy will increase in more polar solvents due to the relative destabilization of this slate by unfavorably oriented solvent dipoles. This result demonstrates that the "blue-shift" of the n pi* State in polar solvents, which is commonly attributed to the effect of hydrogen bonding, can arise entirely from a general solvent effect. The energy of a radiationless vibronic transition from the pi pi* state to the n pi* state will increase in more polar solvents, provided that the solvent does not rearrange during the transition. Consequently, the efficiency of fluorescence quenching by vibronic coupling between the pi pi* and n pi* states is predicted to decrease significantly in such solvents. The geometry of the fluorescent emitting state, obtained by CASSCF optimization of the pi pi* state, is moderately buckled due to the occupation of an antibonding orbital localized to C6. This buckling implies an out-of-plane vibration during the relaxation of the pi pi* state, which is required for vibronic coupling between this state and the n pi* state. Such a solvent-sensitive intramolecular quenching mechanism may account for the observed dependence of the fluorescence lifetime of 2-aminopurine on the local environment both in pure solvents and in DNA.