The title molecule, C19H18N4O2 {systematic name: (RS)-6-[2-(4-methoxyphenyl)-1H-benzimidazol-5-yl]-5-methyl-4, 5-dihydropyridazin-3(2H)-one}, adopts an extended conformation. The dihedral angles between the central benzimidazole ring sytem and the pendant methoxyphenyl and pyridazinone residues are 1.41(18) and 9.7(3)°, respectively. In the crystal, N - H?N hydrogen bonds link the imadazole groups into [001] chains, and pairs of N - H?O hydrogen bonds link the pyridazinone groups into dimers. Together, these generate a two-dimensional supramolecular structure parallel to (010). The layers are linked by C - H?? interactions.