In the present study we have examined the effect of treatment with benz(a)-anthracene (BaA), pyrene (P) and fluoranthene (Fl) on the total liver microsomal content of cytochrome P-450 and elimination of these PAH from blood in rats. The experiment was carried out on adult male rats Wistar breed. Oil solution of inducers: benz(a)anthracene, pyrene and fluoranthene in dose 20 mg/kg were administered individually one or threefold by gavage. 24 h after a single dose of PAH cytochrome P-450 content in rat liver determined by the method of Omura and Sato was the same as in control group. When PAH were administered p.o. for three consecutive days in the fourth day of experiment cytochrome P-450 content was increased from about 0.68 nM/mg protein to about 1.2 nM/mg. Since the increase of cytochrome P-450 may influence on elimination of xenobiotics we have determined affect PAH on their blood level after i.v. administration. One hour after BaA administration and two hours after Fl and P blood samples were collected. Concentration of PAH in blood was determined by gas-liquid chromatography. Pretreatment of rats with BaA decreased elimination of Fl, P, BaA from blood in comparison to the control rats. It indicated on the self-inducing effect of BaA on its own metabolism and inducing effect of metabolism other PAH. Pretreatment of rats with pyrene decreased elimination of BaA. It seemed that pyrene inhibited the oxidation of BaA in rat liver microsomes.