Irradiation of 4-ethoxy-2-methyl-5-morpholino-3(2H)-pyridazinone (2) (Emorfazone) with ultraviolet (UV)-visible light in the presence of 1, 3, 7, 9-tetrabutyl-2, 4, 6, 8(1H, 3H, 7H, 9H)-pyrimido-[5, 4-g]pteridinetetrone 5-oxide (1a), followed by chromatography of the reaction mixture, resulted in the isolation of 5-(2, 3-dihydro-1, 4-oxazin-4-yl)-4-ethoxy-2-methyl-3(2H)-pyridazinone (3), 4-ethoxy-5-(2-hydroxyethylamino)-2-methyl-3(2H)-pyridazinone (4), 4-ethoxycarbonyl-2-methyl-4-morpholino-3(2H)-pyrazolone (5), 4, 4'-bi[4-ethoxycarbonyl-2-methyl-3(2H)-pyrazolone] (6) and 1, 3, 7, 9-tetrabutyl-2, 4, 6, 8(1H, 3H, 7H, 9H)-pyrimido[5, 4-g]pteridinetetrone (1b). The product 3 is produced as a result of photochemical dehydrogenation of 2 by 1a. The formation of the morpholine-ring-opening product 4, one of the metabolites of 2, was proved to occur via the autoxidation of 3 and subsequent stepwise ring cleavage in the presence of oxygen and moisture. The formation of the novel ring-contracted product 5 involves oxygen-atom transfer from 1a to 2 in an excited state. The dimer 6 was shown to be a secondary product formed by further photolysis of 5.