Potentially tautomeric 1,2-dihydro-5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazine-1-ones and their fixed tautomeric forms have been studied, in order to predict their tautomeric equilibrium constants and pK(a) values, using semiempirical PM3, AM1 quantum-chemical calculations at the SCF level in the gas phase and in aqueous solution. The effect of alkylation on the pK(a) value was also investigated. In both the gas phase and in aqueous solution, oxo forms have been found to be more stable than hydroxy and zwitterionic structures. The results obtained from the tautomeric equilibria and acidity calculations are in good agreement with experimental data. (C) 1998 Elsevier Science B.V.