1, 2-Dihydro-6H-pyrrolo [3, 4-d] pyridazine-1-ones exist predominantly as such (104-47: 1) in equilibrium with the 1-hydroxypyridazine form, whereas 1, 2, 3, 4-tetrahydro-6H-pyrrolo-[3, 4-d] pyridazine-1, 4-diones in equilibrium with the monohydroxy-oxo tautomeric forms are preferred by a factor of only ca. 100: 1. It was not possible to determine the position of equilibrium between the 1-hydroxy-6H-pyrrolo [3, 4-d] pyridazine-4-one and the 1, 4- ...