AbstractThe synthetic chemistry of the relatively unknown pyridazino [4,5‐d]pyridazine ring system has been extended. 1,4‐Diaminopyridazino [4,5‐d]pyridazine (VIII) has been prepared by two routes, the most interesting of these being the one‐step conversion of 4,5‐dicyanopyridazine into VIII with hydrazine. Upon nitration VIII gave only the mononitramine (X). Attempts to prepare 1,4‐dichloropyridazino [4,5‐d]pyridazine gave only 4‐chloro‐2H‐pyridazino [4,5‐d]pyridazin‐1‐one (XII). Pyrimido [4,5‐d]pyridazine‐1,3‐dione (XIV) was prepared from pyridazine‐4,5‐dicarboxamide (IV). The hydrolysis of 5,8‐dichloropyrazino [2,3‐d]pyridazine (XV) gave 5‐chloropyrazino [2,3‐d]pyridazin‐8‐one (XVII) and likewise the ammonolysis of XV gave 5‐amino‐8‐chloropyrazino [2,3‐d]pyridazine (XX). As expected the hydrolysis of 5,8‐dibromo‐pyrazino [2,3‐d]pyridazine (XXI) gave 5‐bromopyrazino [2,3‐d]pyridazin‐8‐one (XXII). Attempted catalytic dechlorination of 5‐chloropyrazino [2,3‐d]pyridazin