New 1,2,3,6-tetrahydro-5-nitro-pyrimidine derivatives of the formula ##STR1## in which n represents the numbers 0, 1 or 2,R.sup.1 represents hydrogen or optionally substituted radicals from the series comprising alkyl, alkenyl, alkinyl, aralkyl and hetarylalkyl, andR.sup.2 represents dialkylamino, alkoxy, alkenyloxy, aralkoxy or optionally substituted radicals from the series comprising alkyl, alkenyl, alkinyl, cycloalkyl, aralkyl and hetarylalkyl,and their acid addition salts have been found.It has furthermore been found that these 1,2,3,6-tetrahydro-5-nitor-pyrimidine derivatives have unexpectedly high activity as insecticides, nematicides and ectoparasiticides.