Metanil yellow and Orange II are extensively used in various industries and have not been included in the list of permitted food colours because of inadequate safety evaluation data. In this investigation the effect of Metanil yellow and Orange II on the lipid peroxidation in different subcellular fractions of liver was studied in order to understand the site of hepato-toxic potential and whether its blend has an additive, synergistic or antagonistic effect. Parenteral administration of Metanil yellow (80 mg/kg body weight) to rats for 3 days produced a highly significant (p less than 0.001) increase in NADPH-dependent enzymatic lipid peroxidation in nuclear (62%), mitochondrial (104%) and microsomal (54%) fractions. The same dose of Orange II to rats showed a relatively less but significant effect in nuclear (62%), mitochondrial (59%) and microsomal (38%) membranes. The blend of Metanil yellow and Orange II (40 mg + 40 mg/kg body weight) given intraperitoneally for 3 days resulted in either synergistic or additive effect in nuclear (122%), mitochondrial (130%), and microsomal (62%) membranes. The pro-oxidant free or FeSO4/ADP (1 mM/5 mM) or ascorbate (1 mM) dependent non-enzymatic lipid peroxidation in nuclear, mitochondrial and microsomal fractions was found to be enhanced by Metanil yellow to a greater extent as compared to Orange II, while the blend showed a synergistic or additive response. These results suggest that Metanil yellow and Orange II causes hepatic nuclear, mitochondrial and microsomal membrane damage and that the effect may be more severe with the use of blend.