The effects of the crystal environment on the electronic spectral parameters in 9-methyladenine and N-6-methyladenine have been investigated. We have included the electrostatic effects of the crystal environment, a probable source of discrepancy between the experiment and theory, in the semiempirical molecular orbital calculations using the INDO/S method. The fields and potentials at atomic centers of the molecules in the crystal were calculated using the ground-state charge distribution and were included in the INDO/S Hamiltonian in an iterative process, until self-consistency was attained. The crystal field polarizes the ground state, leading to an increase in the net atomic charges, significantly increasing the magnitude of the ground-state dipole moment in N-6-methyladenine, with the direction unaltered. In 9-methyladenine, a significant change was predicted in the direction of the ground-state dipole moment, with only a slight increase in the magnitude. The changes in the ground-state dipole moment depend on the relative directions of the crystal field and the gas phase dipole moment. The predicted gas phase spectra of both molecules are comparable due to the small effects of methyl substitution on the electronic structure. The crystal field introduces mixing of n pi* and pi pi* transitions. This leads to a slight red shift in the energy of the transitions, changes in intensities, and rotation of the transition dipole moment directions. The interactions between the excited states in the crystal were evaluated by a perturbation treatment. Excited-state mixing leads to extensive transfer of intensities and a slight blue shift in the energy of the transitions. The predicted transition moment directions are in general agreement with experiment, although relative intensities differ in some cases, notably in the two lowest energy transitions.