Owing to the popularity of fried foods and wide fast food industry, the typical western diet contains large quantities of oxidised fat. Primary and secondary autoxidation products of polyunsaturated fatty acids as a component of fried foods affect the antioxidant status and promote lipid peroxidation in tissues. The main objective of this study was to investigate the effect of moderate thermoxidised fats on the cellular antioxidant defence system. Moreover, the effect of heated fat in interaction with antioxidants (vitamin E, vitamin C) was of interest, because they protect against lipidperoxidation. Until now, however, there is no knowledge which kind of lipid peroxidation products (primary or secondary) exerted the most influence on the cellular antioxidant defence system. Three experiments were carried out with rats (Sprague-Dawley) and guinea pigs (Dunkin Hartley) to study the effect of different moderate oxidised fats, in interaction with dietary concentrations of vitamins E and C, on the antioxidant status and antioxidant enzymes activity in tissues, the concentration of lipid peroxidation products, the susceptibility to haemolysis as well as the lipoprotein composition and the atherogenicity of LDL. In conclusion, the protection against lipid peroxidation products was realised rather by the antioxidants than by increased activities of antioxidant enzymes. According to the parameters determined, feeding of moderate thermoxidised fats induced oxidative stress, but there was no evidence of oxidative damage in vivo or proatherogenic effects. The effects of thermoxidised fats were independent of the concentration of primary lipid peroxidation products. The high dietary vitamin E and vitamin C level reduced partial effects of thermoxidised fats. According to the parameters determined, there was the evidence of a sparing effect of vitamin E and vitamin C through the dietary supplementation of both vitamins.