The thesis deals with the content of selected phenolic compounds in some species of the genus Amaranthus, in black elderberry (Sambucus nigra L.) and buckwheat (Fagopyrum esculentum M.). Phenolic compounds are a group of natural compounds exclusively vegetable character. Flavonoids represent only one group of phenolic compounds. Flavonoids show many positive biological effects, in particular act as antioxidants. Natural flavonoids may cause to prevent from coronary- heard diseases and other diseases associated with older age. In recent years the increased attention is paid to flavonoid investigation due to its biological effects. For the determination of phenolic substances there were used two independent analytical methods. There are the high-performance liquid chromatography (HPLC) and micellar electrokinetic capillary chromatography (MECC). The MECC method was used for determination rutin and free quercetin. The highest content of rutin was found in leaves of buckwheat (76,400 mg/kg of dry weight) and the lowest content of rutin was determined in buckwheat hulls. The highest content of rutin was observed in teas from buckwheat leaves and inflorescence. This amount of rutin corresponds with rutin content in more than two pills of Ascorutin (the most favourite flavonoid medicament in the Czech Republic) The HPLC method was used for quantitative determination of phenolic acids. The content of free quercetin was monitored in all samples. No free quercetin was found both in plant material and in samples of teas. The ethanolic extract from the elderberry inflorescence didn´t contain any free quercetin. Free quercetin wasn?t found in any further samples of teas, which were prepared by described methods.