In vitro influence of sodium selenite on cytochrome P-450-dependent formation of active oxygen species on lipid peroxidation (LPO) in rat liver microsomes was studied. Sodium selenite (10(-6)-10(-3) M) did not influence rates of NADPH/Fe-induced formation of active oxygen species (O2.-, OH., H2O2) and NAPDH-dependent LPO. Only at 10(-3) M selenite caused significant decrease in production of hydrogen peroxide in microsomes. Data obtained suggest that sodium selenite at physiological concentrations does not influence formation of active oxygen species by cytochrome P-450 and the rate of enzymatic LPO in rat liver microsomes.