The invention relates to a 2-(diaza-bicyclo-alkyl)-pyrimidone derivative represented by formula (I): ##STR00001## wherein: R1 represents a hydrogen atom, a C.sub.1-6 alkyl group or a halogen atom; R2 represents a hydrogen atom, a C.sub.1-6 alkyl group optionally substituted by 1 to 4 substituents selected from a halogen atom, a hydroxyl group, a C.sub.1-4 alkoxy group; a C.sub.1-2 perhalogenated alkyl group, a benzyl group, a phenethyl group, a benzyloxycarbonyl group, a C.sub.1-4 alkoxy carbonyl group, a benzene ring, a naphthalene ring, a quinoline ring, a phthalazine ring, a 5,6,7,8-tetrahydronaphthalene ring, a pyridine ring, an indole ring, a pyrrole ring, a thiophene ring, a benzenesulfonyl group, a benzoyl group, a pyridazine ring, a furan ring and an imidazole ring; each of the benzyl group, the phenethyl group, the benzyloxycarbonyl group, the benzenesulfonyl group, the benzoyl group and the benzene, naphthalene, quinoline, phthalazine, 5,6,7,8-tetrehydronaphthalene, pyridine, indole, pyrrole and thiophene rings being optionally substituted by 1 to 4 substituents selected from a C.sub.1-6 alkyl group, a benzene ring, a halogen atom, a C.sub.1-2 perhalogenated alkyl group, a C.sub.1-3 halogenated alkyl group, a hydroxyl group, a C.sub.1-4 alkoxy group, a nitro group, a cyano group, an amino group, a C.sub.1-6 monoalkylamino group and a C.sub.2-10 dialkylamino group; R3 represents a 2, 4 or 5-pyrimidine ring or a 2, 3 or 4-pyridine ring, the rings being optionally substituted by a C.sub.1-4 alkyl group, C.sub.1-4 alkoxy group or a halogen atom; R4 represents a C.sub.1-4 alkyl group optionally substituted by a hydroxyl group, a C.sub.1-4 alkoxy group or a halogen atom and n represents 1 or 2; or a salt thereof, or a solvate thereof or a hydrate thereof. 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., such as Alzheimer's disease.