N-alkylmaleimides having 2 to 12 carbon atoms in the alkyl group were evaluated as blocking agents of sex pheromone perception by cabbage looper,Trichoplusia ni(Hübner), males. Compounds containing two to six carbons in the alkyl group were the most active. Complete blockage of antennal response, as measured by electroantennogram procedures, was obtained by exposure of males to a dose of 37.5μg/liter of theN-butylmaleimide for 1 hr. Blockage, presumably reaction with essential sulfhydryl groups, was irreversible, and pheromone offered no protection to maleimide-sensitive sites. Major reduction of male antennal response was necessary to disrupt the insects' ability to locate pheromone in a wind tunnel. Males' interest in mating was apparently destroyed, but females appeared to mate normally following exposure of moths to blocking doses of the chemical.