Phthalophos, organophosphoric pesticide, in the perfused isolated liver of rats transforms to water-soluble oxymethyl phthalimide and phthalimide. The induction of the liver hydroxylating enzymic system with milbex accelerates transformation of phthalophos: in 5 min only 14% of its initial amount is found in perfusate, while in the control--52%. The inhibition of the hydroxylating enzymic system with tetramethylthiuramdisulphide (dithiocarbamate pesticide) inhibits transformation of phthalophos: in 15 min perfusate contains 33% of its initial amount and the control--16%. Induction and inhibition of the hydroxylating enzymic system of the liver affect essentially the phthalophos toxicodynamics, decreasing the degree of the cholinesterase activity.