Reaction of 5, 8-dimorpholino-2-phenylpyrimido [4, 5-d] pyridazine (I) with Grignard reagent or organolithium afforded 4-substituted-3, 4-dihydro derivatives (II). The use of an excess of the reagent gave rise to further substitution at 8-position to afford 4, 8-disubstituted-3, 4-dihydro derivatives (III and VII). Similar results were observed in the reaction of Grignard reagents with analogous tetraazanaphthalene derivatives, 5, 8-dimorpholino-2-phenylpyrimido [4, 5-d] pyrimidine (XX) and 5, 8-dimorpholino-2-phenyl-pyrazino [2, 3-d] pyridazine (XXII), to afford 4-substituted-3, 4-dihydro derivatives (XXI) and 3-substituted-3, 4-dihydro derivatives (XXIII), respectively. Among the obtained derivatives, IIa and IIj exhibited significant diuretic activity.