The invention relates to a pyrimidone derivative represented by formula (I) or a salt thereof wherein X represents two hydrogen atoms, a sulfur atom, an oxygen atom or a C.sub.1-2 alkyl group and a hydrogen atom; Y represents a bond, an ethenylene group, an ethynylene group an oxygen atom, a sulfur atom, a sulfonyl group, a sulfoxide group, a carbonyl group, a hydroxyiminomethylene group, a dioxolan group, a nitrogen atom being optionally substituted; or a methylene group optionally substituted; or a methylene group optionally substituted; R1 represents a 2, 3 or 4-pyridine ring optionally substituted; R2 may represent a C.sub.1-6 alkyl group optionally; a C.sub.3-6 cycloalkyl group, a C.sub.1-4 alkythio group, a C.sub.1-4 alkoxy group, a C.sub.1-2 perhalogenated alkyl group, a C.sub.1-3 halogenated alkyl group, a phenylthio group, a benzyl group, a benzene ring, an indan ring, a 5,6,7,8-tetrahydronaphthalene ring, a naphthalene ring, a pyridine ring, a pyrrole ring, a thiophene ring, a furan ring or an imidazole ring; R3 and R4 represent, each independently, a hydrogen atom, a C.sub.1-6 alkyl group, a hydroxy group, a C.sub.1-4 alkoxy group or a halogen atom; R5 represents a hydrogen atom, a C.sub.1-6 alkyl group or a halogen atom; with the proviso that when R3 and R4 represent each a hydrogen atom then R5 is not a hydrogen atom. The invention relates also to a medicament comprising the said derivative or a salt thereof as an active ingredient which is used for preventive and/or therapeutic treatment of a neurodegenerative disease caused by abnormal activity of GSK3.beta. or GSK3.beta., and cdk5/p25, such as Alzheimer disease ##STR00001##