Fe(II)-induced lipid peroxidation was measured at 25℃ by the TBA method in rat liver mitochondria 3 days after 800R of whole-body irradiation, and decrements in Fe(II) concentrations in the medium were measured by the Nitroso-PSAP method, simultaneously. An induction period (lag) was observed in the reaction curve of the Fe(II)-induced lipid peroxidation. The Fe(II) concentration decreased gradually during the lag period, but decreased rapidly thereafter. The oxidating velocity of Fe(II) in the presence of mitochondria was significant compared with the Fe(II) auto-oxidation in the mixture. At a given dose of Fe (II), a greater amount of mitochondrial protein allowed a shortened lag and accelerated the oxidating velocity of Fe (II). The relationship between MDA formation per unit protein and lag time showed the same tendency, so that MDA formation increased with a lengthened lag. With given doses of protein (0.5mg/ml) and Fe(II), a shorter lag and greater MDA formation were found in irradiated rat liver mitochondria than in non-irradiated rat liver mitochondria. With the addition of Fe(II) (0.10mM), the lag was lengthened and MDA formation increased the same in irradiated as in non-irradiated mitochondria, compared with Fe(II) (0.05mM).