Several excited singlet electronic states of purine nucleobases and related derivatives have been calculated using high-level multireference perturbation theory methods. Purine derivatives with one or two amino or carbonyl groups substituted at positions C(2) and/or C(6) of the purine ring have been included in the study. The effect of the substituents on excited-state energies and wave functions is examined. Some trends have been observed, such as the fact that substitution at the C(2) position decreases the energy of the first pi --> pi* state considerably. Although basic qualitative features of the effects can be explained with the simple frontier molecular orbital theory, ab initio calculations are required to describe the effects quantitatively.