St. John's Wort (Hypericum perforatum L.) was extracted with supercritical carbon dioxide (SC CO2) under 10, 15 and 20 MPa and at 313 and 323 K. The influence Of SC CO2 density (400-800 kg/m(3)) was examined with respect to extraction yield as well as to hyperforin and adhyperforin content, which was determined using a highly selective LC/MS/MS analytical method. A high concentration (up to 526 mg/g total extract) of two phloroglucinol derivatives was determined in this study showing that the absolute extracted amount of hyperforin and adhyperforin (mg per I g of dry plant) is almost constant not depending on total yield of extract, i.e. on density Of SC CO2. Determination of the rate of hyperforin and adhyperforin extraction at 313 K and 10 MPa showed that the maximal rates correspond to consumption of 7-8.5 9 Of CO2 per I g of dry St. John's Wort. Investigation of ultrasonic extraction of St. John's Wort followed after pre-treatment of plant material (323 K; 10 MPa; 2 h; without CO2 flow) indicated that obtained extract contains a higher amount (mg/g plant material) of two most important pharmacological active compounds of St. John's Wort, hyperforin and hypericin, than the extract obtained by ultrasonic extraction only. (C) 2008 Elsevier B.V All rights reserved.