This paper presents evidence of studies on the effects of the narcotic alkaloids, cocaine hydrochloride, morphine sulfate, and codeine phosphate, on nonenzymatic lipid peroxidation in rat brain mitochondria. These organelles abound in polyunsaturated fatty acids and are thus susceptible to oxidative attack. Lipid peroxidation was indexed mainly by assaying the extent of malonaldehyle (MDA) production and also the formation of fluorescent products in the course of the reaction. We found that morphine sulfate lowered fluorescence while the other two alkaloids showed no effect on lipid peroxidation in the absence of the inducers, 1.0 mM ascorbic acid or 0.1 mM ferrous sulfate. The apparent antioxidative nature of morphine sulfate was also observed in its effects on induced and noninduced MDA production, both cocaine hydrochloride and codeine phosphate stimulated MDA production by about 20% in the absence of any inducers. This paper also attempts to draw a structure-activity relationship for the antioxidative action of opium alkaloids, which we postulated to be due to the chelating capability of the alkaloid molecule.