The electronic structure of pteridines and N-methylpteridinium cations was calculated in terms of the CNDO/2 approximation. The obtained data on the electron density distribution in the molecules of pteridines and also the energy characteristics of the pteridinium cations make it possible to predict the outcome of quaternization. The results from the calculations are compared with the experimental data. It is shown that the direction of nucleophilic attack in N-alkylpteridinium cations correlates with the total charge of the fragment of the covalently bonded atoms.