1-alkyl-2-(bromomethyl)aziridines were transformed into 1-alkyl-2-(N-alkyl-N-ethylaminomethyl)aziridines upon treatment with 2-3 equiv. of methyllithium in THF or Et(2)O; the peculiarity in this transformation comprises the presence of an N-ethyl group in the end-products as well as the total number of carbon atoms, resulting from a highly unusual reaction course with a novel S(N)2'-type substitution at the aziridine moiety and liberation of acetylene from an intermediate vinylamine as the key reaction steps.