To explore pharmacokinetic properties of prenylflavonoids from the Traditional Chinese Medicinal plant Epimedium, three doses of a standardized extract (100, 300 and 600 mg/kg body weight), were administered to ovariectomized rats and serial blood samples were obtained. Serum concentrations of the Epimedium prenylflavonoids icariin, icariside I, icariside II, icaritin and desmethylicaritin were determined by LC-MS/MS. Aliquots of sera were also applied to human cell lines that permanently express ERalpha and ERbeta proteins for the ex vivo measurement of estrogenic activity. All five prenylflavonoids exhibit non-linear dose-dependent increases in the area under concentration versus time curves. Two distinct pharmacokinetic patterns were evident, an early phase wherein icariin and icariside II reached t(max) 0.5-1 h, and a late phase wherein icariside I, icaritin and desmethylicaritin peaked at t(max) 8h. Total concentrations of icaritin and desmethylicaritin reached C(max) approximately 2 microM and approximately 0.25 microM respectively. Estrogenic activity in Epimedium-treated rat sera lagged by several hours compared to animals treated with control drug estradiol benzoate, corresponding to the appearance of bioactive metabolites desmethylicaritin, icaritin and icariside I. Following glucuronidase/sulphatase treatment, prolonged estrogenic activity at higher Epimedium doses (300 and 600 mg/kg of body weight) was evident, and correlated with the persistence of micromolar levels of icaritin at the 48-72 h sampling period. The depot effect resulted in time-concentration bioactivity profiles at the three Epimedium doses (area under curve 374, 543, and 771pME2 h(-1)) that exceeded that observed for estradiol benzoate (148pME2 h(-1)). Our study correlated the pharmacokinetics of prenylflavonoids with the dynamics of their estrogenic effects and reveals the potential estrogenicity of this Epimedium extract. This study may aid the development of prenylflavonoids as drugs for menopause and other conditions requiring estrogenic action.