Pyrimidinedione derivatives of the formula: ##STR1## wherein R.sup.1 and R.sup.2 is so linked with each other as to make an alkylene chain and thus form a heterocyclic structure, A, R.sup.3, R.sup.4, X.sup.1, X.sup.2 and X.sup.3 are substituents, respectively, and n is 2 or 3 have a basic backbone in which a phenyl group part and a pyrimidinedione part are linked by a structure comprising an alkyl chain and a heterocyclic ring having two nitrogen atoms. These compounds are useful for a medical treatment of cardiac arrhythmias.