The rats (100 mg/kg, once a day, per os, during 3 days) were administered suspension of benzonal in starch gel before irradiation of 12 Gy. Induced and uninduced rats were irradiated on the following day after stopping benzonal administration and were decapitated at 10, 12, 15 and 21 o'clock during the first and second day after irradiation and also on the fourth day (day of mass death of irradiated rats). It has been established that irradiation changes the dynamics of cytochrome P-450 concentration in microsomal fraction of rat liver. The essential decrease of the content of cytochrome on the second day after irradiation was accompanied by intensification of the process of its inactivation, but stoichiometry between the decrease of P-450 and the increase of cytochrome P-420 was not observed. The high inducing and stabilizing effects of benzonal on cytochrome P-450 and on the liver persisted. In comparison with irradiation the unfavourable effect of benzonal on immunocompetent organs (thymus, spleen) was not found.