A general route for the synthesis of a novel class of pyridazin-3-one derivatives 3 by the reaction in acetic anhydride between 3-oxo-2-arylhydrazonopropanals 1 and some active methylene compounds like p-nitrophenylacetic acid and cyanoacetic acid was established. Under these conditions the pyridazin-3-one derivatives 3 were formed as the sole isolable products in excellent yield. The 6-acetyl-3-oxopyridazine derivative 3l was reacted with DMF-DMA to afford the corresponding enaminone derivative 4, which reacts with a variety of aminoazoles to afford the corresponding azolo[1,5-a]pyrimidine derivatives 5-7. Also, in order to explore the viability and generality of a recently uncovered reaction between 3-oxo-2-arylhydrazonopropanals and active methylene compounds, a variety of 2-amino-6-aryl-5-arylazo-3-aroylpyridines 16-19 were prepared by reacting 3-oxo-2-arylhydrazonopropanals with miscellaneous active methylene compounds like 3-oxo-3-phenylpropionitrile, hetaroylacetonitriles and cyanoacetamides. These 2-aminopyridine derivatives undergo smooth reactions with cyanoacetic acid that led to the formation in high yield of a new class of 1,8-naphthyridine derivatives 24. The structures of all new substances prepared in this investigation were determined by the different analytical spectroscopic methods, in addition to the X-ray crystallographic analysis.