Novel quinoxaline-di-N-oxide derivatives of the general formula ##STR1## in which R.sub.1 is hydrogen or straight-chain or branched-chain alkyl having 1 to 4 carbon atoms,R.sub.2 and R.sub.3 independently of one another are each straight-chain or branched-chain alkyl having 1 to 4 carbon atoms, or together with the N atom they form a heterocyclic ring which is unsubstituted or is substituted by alkyl having 1 to 4 carbon atoms, and which has 4 to 5 ring carbon atoms and optionally an oxygen atom as further hetero atom,R.sub.4 is hydrogen, methoxy, methylthio, hydroxyl, fluorine, chlorine, bromine or cyano,A is alkylene having 1 to 4 carbon atoms, with the proviso that if R.sub.4 has a meaning other than cyano, A is 1,2-ethylene, and R.sub.1 is hydrogen,including the acid addition salts thereof; processes for producing them; compositions containing these compounds; and the use thereof. The novel active substances serve to combat pathogenic microorganisms in the field of veterinary medicine. They are effective in the case of diseases of the respiratory tract in poultry, and infections of the intestinal tract and of the urogenital system in domestic animals and productive livestock. They are in addition suitable as a feed additive for promoting the growth of animals. Depending on the type of application, the novel active substances may be administered perorally or via the abomasum or directly by injection, the active substances being applied on their own or in combination with customary inert carriers, either in the solid form or in the liquid form.