9-(Substituted thio)-4H-pyrido[1,2-a]pyrimidin-4-ketone derivatives of the formula(I)wherein n is 0 or 1, R is -COR<1>, -CONR<4>R<5> or -CH2R<6>, R1 is C1-C5 alkyl, C3-C7 cycloalkyl, allythio, styryl, phenoxymethyl, thienylmethyl, C6-C10 aryl optionally substituted, benzyl optionally substituted or 5- or 6-membered heterocyclic group optionally substituted, R<2> and R<3> each is hydrogen, C1-C5 alkyl, carboxy, C2-C6 alkoxycarbonyl or benzyloxycarbonyl optionally substituted, R<4> and R<5> each is hydrogen, C1-C5 alkyl, C3-C7 cycloalkyl or phenyl optionally substituted, and R<6> is pyridyl or phenyl optionally substituted)being useful as antiulcer agents are disclosed. |