The invention relates to a pyrimidone derivative represented by formula (I) or a salt thereof wherein: X represents two hydrogen atoms, a sulphur 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 sulphur atom, a sulfonyl group, a sulfoxide group, a carbonyl group, a nitrogen atom being optionally; or a methylene group optionally substituted; R1 represents a pyrimidine group optionally substituted; R2 may represent a C.sub.1-6 alkyl group optionally substituted by a C.sub.6,10 aryloxy or a C.sub.6,10 arylamino group; a C.sub.3-6 cycloalkyl group, a C.sub.1-4 alkylthio 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 represents each independently a hydrogen atom, 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. 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.3 or GSK3.beta.3 and cdk5/p25, such as Alzheimer disease.