The optimal geometries, relative stabilities, dipole moments and ionization potentials of a series of tautomers of uracil, thymine and cytosine have been calculated by the MNDO method. The results are in good agreement with the available experimental data. The optimal MNDO geometry of the “diketo” tautomer of uracil is very close to the recently published optimal ab initio STO-3G geometry. It is expected that the MNDO geometries of other pyrimidine base tautomers could be used in ab initio calculations. The calculated relative stabilities agree with the results of earlier non-empirical studies performed without geometry optimization, although the differences in energy of the tautomers found by MNDO are significantly smaller than those predicted by ab initio methods.