A pyrimidone derivative represented by formula (I) or a salt thereof: ##STR00001## Wherein: R1 represents a hydrogen atom or a C.sub.1-6 alkyl group which may be substituted by a C.sub.6,10 aryl group; R2 represents a C.sub.1-10 alkyl group which may be substituted, a C.sub.2-6 alkenyl group which may be substituted, a C.sub.3-6 alkynyl group which may be substituted, a C.sub.3-6 cycloalkyl group which may be substituted, or a C.sub.6-10 ARYL group which may be substituted; or R1 and R2 form together a C.sub.2-6 alkylene group which may be substituted; or R1 and R2 form together a chain of formula --(CH.sub.2).sub.2--X--(CH.sub.2).sub.2-- or --(CH.sub.2).sub.2--X--(CH.sub.2).sub.3-- where X represents a oxygen atom, a sulfur atom, or a nitrogen atom which may be substituted; R3 represents a 2, 3 or 4-pyridyl group optionally substituted by a C.sub.1-4 alkyl group, C.sub.1-4 alkoxy group or halogen atom; and R4 represents a C.sub.1-10 alkyl group optionally substituted by a hydroxyl group, amino, C.sub.1-6 monoalkylamino group, C.sub.2-12 dialkylamino group or C.sub.6,10 aryl group which may be substituted.And 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.