Modification of the major insecticide fipronil (1) by replacing three pyrazole substituents (hydrogen for both cyano and amino and trifluoromethyldiazirinyl for trifluoromethylsulfinyl) gives a candidate photoaffinity probe (3) of high potency (IC(50) 2-28 nM) in blocking the chloride channel of Drosophila and human beta 3 GABA receptors.