Previous investigations have shown that exposure to DDT results in increased activity in soluble and microsomal enzyme in insects. These responses in DDT-resistant Triatoma infestans nymphs and in adults of DDT-resistant Musca domestica (the common house fly) are accompanied by increased RNA and protein synthesis. The phenomenon has been termed induction. Since induction has not been demonstrated in insecticide susceptible insects, the phenomenon is thought to be associated with resistance. The research reported here was a study of the inductive effect of two other chlorinated organic insecticides on house flies and the relationship between induction and insecticide resistance. The house fly strains studied were resistant to dieldrin and thus could tolerate this material and its precursor, aldrin, when these were applied as inducers. The strains differ in microsomal oxidase activity with Isolan-R highest, Orlando intermediate, and dield; cyw lowest. The flies were treated with inducers by two methods, by 24 hour exposure to deposits of test compounds in glass jars and by topical application of the compounds. The response to the inducers was measured by in vivo and in vitro methods. Mortality tests with the insecticide carbaryl, indicated the in vivo response. Assays of naphthalene hydroxylase and heptachlor epoxidase were used to measure the effect of the inducers on microsomal enzyme activity. The effect of the treatments on protein synthesis was determined by use of a protein synthesis inhibitor, cycloheximide. Incorporation of C¹⁴ -isoleucine into subcellular protein was also followed. Aldrin and dieldrin caused a large increase in microsomal oxidase activity and an increased tolerance to carbaryl. The response was dose dependent. The age of the flies affects the magnitude of response but not the potential to respond. Cycloheximide inhibited both in vivo and in vitro responses to dieldrin. Dieldrin also caused an increased incorporation of isoleucine into microsomal and soluble protein. Additional experiments showed that dieldrin does not stimulate enzyme activity when added during various stages of microsome preparation. BSA, which is reported to neutralize an endogenous inhibitor in microsome preparation, produced an additive effect with dieldrin treated flies. Kinetic studies revealed that dieldrin effects only the amount of oxidase present. It was concluded that aldrin and dieldrin exerted their effect in house flies through increased synthesis of protein. The inductive action of dieldrin in house fly strains appears to depend on the levels of their microsomal oxidase activity, being greatest when microsomes are most active. This suggests that the action of the inducer is at the site of gene regulation of protein synthesis, and results in increased production of detoxication enzymes.