Quantum chemical (AM1) and solvation model calculations have been applied to the study of tautomeric stabilities in three anthocyanidins, so as to single out the dominant tautomeric forms (neutral and anionic) at physiological pH. The theoretical information concerning the tautomeric stabilities and electronic structures of anthocyanidins are essential for the development of quantitative structure-activity relationships. The present results allow us to discriminate between the wide variety of tautomeric forms, and to infer that, depending on the pH, only two neutral tautomers and two anionic tautomers should be present in solution. On interpretative grounds, the main factors determining stability are chiefly ascribable to the extent of pi-electron delocalization and to the possibility of classical resonance structures. Several theoretical descriptors have been calculated for use in quantitative structure-activity relationships.