The protective effect of magnesium as magnesium sulfate (MS) on sodium-metavanadate- (SMV-) induced lipid peroxidation (LPO) under in vivo and in vitro conditions was studied. The 18-week SMV intoxication (Group II, 0.125 V(end)/mL) enhanced spontaneous malondialdehyde (MDA) generation in rat liver, compared with the control (Group I) and MS-supplemented animals (Group III, 0.06 Mg(end)/mL). Coadministration of SMV with MS (Group IV, SMV-MS) caused a return of the MDA level to the control value range. The effect seems to result from the Mg(end)-independent action and its antagonistic interaction with V(end). The in vitro treatment of liver supernatants (LS) obtained from all the tested animals groups with selected exogenous concentrations of Fe(exg) or V(exg) exhibited enhanced MDA production, compared with spontaneously formed MDA. It also showed Mg(exg)-stimulating effect on LPO (LS I, Group I) and revealed that the changes in the MDA generation in LS IV (Group IV) might have resulted from the synergistic interactions of V(end) with Fe(exg) and V(exg) and from the antagonistic interactions of Mg(end) with Fe(exg) and V(exg). The findings allow a suggestion that adequate Mg intake for a specific period in the conditions of SMV exposure may prevent V-induced LPO in the liver.