Incubation of rabbit brain endoplasmic reticulum membranes with either ferrous sulfate/EDTA or ferrous sulfate/EDTA and hydrogen peroxide led to the loss of efficiency of membranes to sequester Ca2+, which did not correlate with changes in conjugated diene formation. The production of practically non-detectable amount of conjugated dienes that occurs during the period of incubation of microsomes with lipid peroxidation initiators represents lipid peroxidation that is enough to produce changes in membrane permeability towards Ca2+. Addition of stobadine was able to prevent Ca2+ transport damage in a dose-dependent manner and drug concentrations higher than 200 microM were able in our model system to confer the defense against free radical and heavy metal initiated lipid peroxidation. The EC50 values for microsomes treated with Fe2+ and Fe2+/H2O2 were 12 microM and 25 microM, respectively. In our model system stobadine seems to be at least as effective as butylated hydroxytoluene, which is considered to be a good chain-breaking antioxidant. In contrast to stobadine alpha-tocopherole acetate was less potent; the effect of 1 mM alpha-tocopherole acetate being comparable to the effect of 20 microM stobadine.