New pyrimido [4, 5-c] pyridazines were synthesized by cyclization of 3-amino-4-carbamoylpyridazines (IV, VII) and 3-chloro-4-ethoxycarbonylpyridazine (II). When 3-chloro-5-hydroxypyrimido [4, 5-c] pyridazine (XVI) was treated with phosphorous oxychloride and N, N-dimethylaniline, 1, 4-dihydro-3, 5-dichloro-4-(4'-N, N-dimethylaminophenyl) pyrimido-[4, 5-c] pyridazine (XX) was obtained, the structure was established by nuclear magnetic resonance spectra of dechlorination and hydrolysis products. Amination of XX affored 3-chloro-5-amino compounds.