The recent years again saw increased interest of the public in the use of traditional natural remedies, such as plant extracts, for the treatment and prevention of diseases. This paper evaluates the antioxidative activity and content of phenolic substances of selected plant extracts from medicinal herbs, sold as dietary supplements with therapeutic effects. Using the production of 3-nitrotyrosine as the marker, the effects of extracts on the inhibition of peroxynitrite-induced nitration of tyrosine was examined, and the results were compared with the ability to extinguish the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). A linear correlation (r = 0.929) was found between the ability to extinguish DPPH and to inhibit tyrosine nitration. The found antioxidative activity ranged between values of 0-1702, or 0-1482 micromol of the catechine equivalent/100 ml of the extract. The total content of phenolic substances oscillated within values of 9.4-268.3 mg of the equivalent of gallic acid/100 ml of the extract. The significant linear relationship between antioxidative activities (r>0.9) and the total content of phenolic substances shows that phenolic substances are the principal group responsible for the antioxidative activity of the extracts tested.