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Potentially Tautomeric 1,2-Dihydro-1-oxo-5H-pyridazino(4,5-b)indole and 3,4-Dihydro-4-oxo-5H-pyridazino(4,5-b)indole. -The title compounds (IIa) and (Xa) can exist together with the corresponding hydroxy analogues of, e.g., (IV) and (VI) in three potentially tautomeric forms for each system. Three additional tautomeric structures due to a mobile hydrogen at the N-atom of the indole are excluded by molecular-orbital calculations. The tautomeric equilibrium constant for, e.g. the system (IIa) can be deduced from the pKa-values of the mono-and diprotonation of (IV) and (VIIIb). The results show that the oxo form is favored over the hydroxy form to a high extent. -(GUEVEN, A.; JONES, R. A.; J.