Ionizing radiation damage to cells is mainly due to the action of very reactive hydroxyl radicals, excited states and free radicals of macromolecules. It represents a class of chemical assaults dependent on the formation in high yields of highly reactive organic radicals. The aim of this work was to study the radiation-induced damage of rat liver microsomal membranes. Rats were singly irradiated with gamma-rays (1 Gy), sacrificed after 1 to 4 days and liver microsomal membranes were isolated. The level of lipid peroxidation products in the microsomal membranes increased (0.59+/-0.05 nmol TBARS/mg protein in comparison with 0.45+/-0.03 nmol/mg protein for the control), whereas the rates of NADPH-oxidation by liver microsomal membranes (1.26+/-0.13 nmol/min/mg protein in comparison with 1.99+/-0.21 nmol/min/mg protein for the control) and NADPH-ferricyanide reduction (168+/-11 nmol/min/mg protein in comparison with 269+/-15 nmol/min/mg protein for the control) decreased after 1 day following the whole body irradiation. At the same time we did not observe any significant changes in the level of microsomal membranous protein SH groups after the irradiation. The TBARS level and the rate of NADPH-oxidation but not the activity of ferricyanide NADPH-reductase were brought back to the control values 4 days after irradiation. The susceptibility of microsomal membranes to the chemically induced oxidative stress (by exogenous organic hydroperoxide (tBHP) treatment) before and after the whole body irradiation of rats was compared. The post-mortal liver microsomal membrane treatment by tBHP drastically changed the membrane structure and enzymatic activities. The TBARS level highly increased and the protein SH-group content decreased after chemically induced oxidative stress. The microsomal membrane rigidity increased after tBHP treatment up to 0.5 mM and slowly decreased at higher oxidant concentrations. These changes were more significant and occurred at lower oxidant concentrations in the microsomes of the irradiated animals after 1 day as compared to the control ones. The microsomal NADPH-oxidase and Fe(3+)-NADPH oxidoreductase activities decreased after the tBHP treatment of the microsomes of non-irradiated animals and either increased or remained unchanged for irradiated rats. Thus, low-dose rat irradiation as well as the microsomal membrane oxidative agent treatment significantly changed the membrane functional properties. The preliminary irradiation increased the membrane susceptibility to the chemically induced oxidative stress.