The reaction of 4-aryl-4-oxo-2-(5-oxo-1, 3-diphenyl-2-pyrazolin-4-yl) butanoic acids with hydrazine hydrate in ethanol afforded the 4-(5-oxo-2-pyrazolin-4-yl)-3 (2H)-pyridazinones and in 1-butanol a pyrazolo [3, 4-c] pyridazino [4, 3-e] pyridazine derivative was obtained while in acetic acid the 6-aryl-3 (2H)-pyridazinone was the product. Probable mechanism has been proposed. The behavior of the 4-(5-oxo-2-pyrazolin-4-yl)-3 (2H)-pyridazinones ...