Certain oximes, including cyometrinil [(Z)-α-(cyanomethoxy)iminobenzeneacetonitrile], CGA-92194 [N-(1,3-dioxolan-2-yl-methoxy)-iminobenzeneacetonitrile], 2-pyridinealdoxime-O-benzyl ether (PA-B), and 2-pyridinealdoxime-O-phenethyl ether (PA-PE), which are known to protect sorghum from chloroacetanilide herbicides, increased the toxicity of the insecticide propoxur (o-isopropoxyphenyl-methyl carbamate) to a resistant strain of housefly (Musca domestica L.). Flurazole [5-thiazolecarboxylic acid, benzyl ester, 2-chloro-4-(trifluoromethyl)], a nonoxime protectant for grain sorghum, had less effect on the toxicity of propoxur than did the oximes. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], which can be used as a herbicide in sorghum [Sorghum bicolor (L.) Moench.] only if the seed have been treated with a protectant, also increased the toxicity of propoxur to houseflies. Commercially available insecticide synergists increase insecticide activity by inhibiting NADPH-dependent microsomal mixed-function oxidase activity. Sorghum coleoptiles possessed the enzyme system necessary to oxidize aldrin (1,2,3,4,10,10-hexachloro-1,4,4a,5,8,8a-hexahydro-1,4-endo-exo-5,8-dimethanonaphthalene) to dieldrin (1,2,3,4,10,10-hexachloro-6, 7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-1,4-endo-exo-5,8-dimethanonaphthalene), which is a standard test for oxidative metabolism in insects. Under the conditions of these tests, the aldrin epoxidase activity in sorghum coleoptiles was not affected by pretreatment with cyometrinil or flurazole.