The condensation of 2-(2,6-dichlorophenyl)acetonitrile (I) with 3,6-dichloropyridazin-3-yl)acetonitrile (III), wich is condensed with 2,4-difluorothiophenol (IV) by means of NaH in THF to yield the thioether (V). The partial hydrolysis of the cyano group of (V) with conc. H2SO4 at 100 CC affords the acetamide (VI), wich is finally cyclized with N,N-dimethylformamidedimethylacetal (VII) in refluxing toluene te provide the target pyrimido-pyridazine derivative.