A reaction of ethyl α-diazo-β-oxo-5-(4-chloro-2-methylthiopyrimidine) propionate (III) with hydrazine hydrate in ethanol afforded 1, 2-dihydro-4-hydroxy-7-methylthiopyrimido-[4, 5-c] pyridazine-3-carboxamide (VII), of which structure was determined by chemical and infrared, ultraviolet, nuclear magnetic resonance and mass spectral evidences : VII was acetylated to 1, 2-diacetyl-4-hydroxy-1, 2-dihydro-7-methylthiopyrimido [4, 5-c] pyridazine-3-carboxamide (XI) and 4-acetoxy-9-acetyl-1-methyl-7-methylthio imidazo [3', 4' : 2, 3]-pyridazo [6, 5-d] pyrimidin-3 (9H)-one (XII), which were interconvertible. Both XI and XII were converted to 9-acetyl-4-hydroxy-1-methyl-7-methylthioimidazo [3', 4' : 2, 3]-pyridazo [6, 5-d] pyrimidin-3 (9H)-one (XIII). The hydrolyzed product (X) of VII was treated with alkaline permanganate to give 6-methylthio-1H-pyrazolo [3, 4-d] pyrimidine-3-carboxylic acid (XIV), which was subjected to decarbonylation and desulfurization. The formation mechanism of VII was discussed. Treatment of the acid (X) with benzoyl peroxide in ethanol in the presence of triethylamine afforded ethyl 1, 4-dihydro-7-methylthio-4-oxopyrimido [4, 5-c] pyridazine-3-carboxylate (XVII). The corresponding acid (XVIII) is a key intermediate for syntheses of 1, 4-dihydro-4-oxopyrimido [4, 5-c] pyridazine-3-carboxylic acids, which would be expected as antibacterial agents.