The behaviour of Mg related to vanadium(V)-induced lipid peroxidation (LPO) under in vitro conditions was examined. The studies performed on the liver supernatants (LS) obtained from control, sodium metavanadate-intoxicated, and sodium metavanadate-magnesium sulphate-administered male Wistar rats revealed and confirmed the pro-oxidative potential of V. Simultaneously, they indicated that the improved Mg status may be one of the mechanisms by which the treatment with this element may contribute to reduction of oxidative stress under the conditions of vanadate exposure. On the other hand, the results confirmed that Mg may also stimulate LPO and demonstrated that the incubation conditions and the experimental treatment of the rats from which the liver supernatants were obtained affect the intensity of the examined free radical process.