The effects of salt concentration gradient (inside to outside) on the lipid peroxidation of porcine intestinal brush-border membrane vesicles have been studied and several interesting features of the peroxidation have been elucidated. The addition of dithiothreitol and Fe2+ is far more effective in induction of the lipid peroxidation than any of the other metal ion species tested (Fe3+, Cu2+, Ni2+, Zn2+ and Cr3+). The peroxidation rate of the membrane vesicles induced by dithiothreitol plus Fe2+ was sensitive for the incubation temperature and was increased with increase of the temperature. Imposition of an inward salt concentration gradient on the membrane vesicles preloaded with 300 mM mannitol by addition of 100 mM chloride of K+, Na+, Li+, Rb+, NH4+ or choline to medium produces a very large reduction of the lipid peroxidation induced by dithiothreitol plus Fe2+. The membrane peroxidation is depressed more with the mannitol (300 mM)-preloaded vesicles than with the K2SO4 (100 mM)-preloaded vesicles when they are incubated in medium containing 20-100 mM of K2SO4. Addition of membrane-permeant anions such as SCN- and I-, but not addition of NO3-, to incubation medium has been found to decrease markedly the lipid peroxidation of the mannitol-preloaded vesicles. From these results it is suggested that the lipid peroxidation of the brush-border membranes by addition of dithiothreitol plus Fe2+ is sensitively changed with change in ionic strength.