The effect of a single and multiple intravenous injections of a nanosized magnetite suspension on total prooxidant and antioxidant activity of blood plasma has been investigated by the method of luminol-dependent chemoluminescence. Magnetite nanoparticles possess dose-dependent prooxidant properties due to their iron atoms and at the same time their trigger compensatory activation of antioxidant systems in the rat blood plasma. After a single intravenous administration of magnetite the studied parameters of blood plasma returned to the normal level by the end of the experiment as due to removal ofnanoparticles from the body. In the case of multiple administration of the magnetite suspension dose-dependent changes in the pro- and antioxidant plasma activity persist during the whole experiment. Accumulation of magnetite particles in the cells of the mononuclear phagocytic system in the rats' liver, lungs and kidneys is associated with hemodynamic damages, local dystrophic and necrotic changes of parenchyma in these organs. After a single intravenous injection magnetite nanoparticles are identified in the rat organs for 40 days, but their number decreases by the end of the experiment.