Experiments carried out with ethanol plant extract 1 (Asclepias syriaca L.). 2 (Astragalus onobrychis L.) and 3 (Chenopodium ambrosiodes L.) applied to lard and bleached sunflower oil as substrates showed their antioxidative activity. The increase of extract concentration and the addition of citric acid or lecithin as synergists increase the inductive period. The comparison of oxidative stability of sample with 0.01% BHA, control sample and samples with the investigated extracts showed that the extract effect, even at significantly high concentration (0.2%), is several times lower than those of commercial antioxidants. The addition of synergists improves the effect of extract, but it does not change the pattern of antioxidative activity. The use of highly unsaturated substrate, completely bleached sunflower oil, indicates for plant extract 3 practically the same order of inductive periods. The mentioned decrease in differences is not caused by significant increase of antioxidative activity of the extract itself, but by lower activity of BHA in the substrate which indicates the importance of careful selection of antioxidative materials for different substrates.