8-Oxo-7, 8-dihydropyrido[2, 3-d]pyridazine-5-carboxylic acid, 7-phenyl-8-oxo-7, 8-dihydropyrido[2, 3-d]pyridazine-5-carboxylic acid, and their esters, amides and hydrazides were prepared. Alkaline hydrolysis of 7-phenyl-8-oxo-7, 8-dihydroprido[2, 3-d]pyridazine-5-carboxylates gave phenylhydrazone of 3-formylpicolinic acid and good yields of 3-(2-carbamoyl)pyridylglyoxylates. Reaction of quinolinic anhydride with 2 molar equivalents of phenylhydrazine in acetic acid gave the cyclized phenylhydrazide, and its structure, 5-hydroxy-7-phenylpyrido[2, 3-d]pyridazin-8(7H)-one, was determined by its chlorination followed by catalytic hydrogenation to give 7-phenyl-1, 2, 3, 4-tetrahydropyrido[2, 3-d]pyridazin-8(7H)-one. 6, 7-Diphenylpyrido[2, 3-d]pyridazine-5, 8(6H, 7H)-dione was also prepared.