The dynamics of toxicity of insecticides to insects are examined. The relationship between insect toxicity and the rates of penetration and detoxication of insecticides can be expressed by simple mathematical equations or by graphical evaluation. Results of calculation of published data on house flies, Musca domestica L., topically treated with different insecticides indicate that the difference in the integrated area between penetration and detoxication represents the accumulation of toxicants, the amount of which determines the difference in toxicity to different species of insects as well as to the susceptible and resistant strains of the same species. Additional data indicate the relative importance of penetration and detoxication of several insecticides tested on house flies by topical application, injection with and without synergist, infusion, and spray methods.