The condensation of 1-amino-1,2-dihydro-3,6-pyridazinedione (1) and 2-amino-2,3-dihydro-1,4-phthalazinedione (2) with 1,3-diketones or 1,3-dialdehydes in polyphosphoric acid gave 6,9-dihydro-6,9-dioxopyridazino [1,2-a][1,2,3]triazines (3-6) and 6,11-dihydro-6,11-dioxo[1,2,3]triazino[1,2-b]phtha lazines (7-10), respectively. The condensation of 2 with 2,4-pentanedione in acetic acid gave N-alkylidene intermediate (11), which was cyclized to 9 by treatment with polyphosphoric acid.