The aim of this study was to set up a simple procedure for assessing lipid peroxidation (L.P.) and testing the activity of antioxidant compounds. L.P. was determined in rat brain homogenates by measuring the endogenous and stimulated accumulation of malonaldehyde (MDA). MDA was assayed by an HPLC method. Homogenates spontaneously formed appreciable amounts of MDA. The addition of increasing concentrations of FeCl2 resulted in a linear accumulation of MDA, up to 16.6-fold at 50 microM. An organic form of iron (Fe-saccharate) was less active on MDA formation (11.4-fold increase at 100 microM). The addition of xanthine-xanthine oxidase resulted in only a 2.4-fold increase in MDA formation. Various antioxidant or chelating compounds effectively inhibited L.P., with IC50 between 0.1 microM (phenoxazine) and 4-50 microM (alpha-tocopherol). Their potencies depended on the iron concentration and time of preincubation with the homogenates. In conclusion, this is a simple and reliable procedure for studying L.P. and inhibiting agents, provided that the experimental conditions are carefully assessed.