The invention relates to new heterocyclic S-imino-S-oxides of the formula I ##STR1## in which one of the symbols X.sub.1 and X.sub.2 denotes a direct bond and the other denotes the vinylene group --CH=CH-- or the epithio radical --S--, Y.sub.1 and Y.sub.2 conjointly denote an additional bond or one of the symbols denotes hydrogen and the other denotes hydrogen or, conjointly with the symbol R.sub.1 or R.sub.2, which is present on the same carbon atom, denotes the oxo radical, one of the symbols R.sub.1 and R.sub.2 denotes hydrogen or lower alkyl and the other denotes hydrogen or lower alkyl or, conjointly with the symbol Y.sub.1 or Y.sub.2, which is present on the same carbon atom, denotes the oxo radical, or, if Y.sub.1 and Y.sub.2 conjointly denote an additional bond, also denotes lower alkoxy, and R.sub.3 and R.sub.4 independently of one another denote hydrogen, halogen up to atomic number 35, lower alkyl, lower alkoxy or trifluoromethyl and R.sub.5 denotes hydrogen or lower alkyl, and to their acid addition salts in particular the pharmaceutically with inorganic and organic acceptable acid addition salts. These new compounds possess valuable pharmacological properties. In particular they have an anticonvulsive activity and are useful for the treatment of epilepsy and of states of tension and states of agitation.