There is a potential hazard in mixed intoxications by pyrethroids and organophosphate insecticides, due to the fact that low toxicity of pyrethroids on mammals is chiefly due to quick cleavage of molecule by esterases, which can be thwarted by esterase inhibitors. We have developed a method in order to measure the duration and the intensity of potentiation of deltamethrin by a variety of organophosphate compounds. It was demonstrated that some of them (azinphos, dichlorvos, dimethoate, fenitrothion, omethoate) induce an increase of toxicity of deltamethrin. But, the total toxicity of association of Deltamethrin with Dimethoate, Fenitrothion, is weak, and does not prohibit their use. Others (methyl-parathion, acephate, phosphamidon, monocrotophos) have no such effects, even if they have a very high intrinsic toxicity. Cholinesterase inhibitors of the carbamate group are ineffective. It is suggested that the potentiation is mainly in relation with the kinetic of esterase inhibition, which is different, and specific to each organophosphate compound. So, it is essential that a specific toxicological measurement must be performed with any different insecticide, in order to anticipate the danger of a mixed intoxication by pyrethroids and organophosphates.