A validated method for the quantitation of trace levels of quercetin from human plasma to be used in pharmacokinetic and biomarker studies is presented. Quercetin conjugates were hydrolysed enzymatically, plasma proteins were removed using a Bond Elut C18 extraction column and additional interferences were removed by extracting them into a toluene-dichloromethane mixture. The HPLC system consisted of an Inertsil ODS-3 column (250 x 4.0 mm) and a mobile phase with 59% methanol in phosphate buffer (pH 2.4). High selectivity and a low quantitation limit (0.63 microg/l) were achieved by using electrochemical detection at a low potential. The method has excellent reproducibility: R.S.D. values of peak-heights were 2% and 7.9%, respectively, for within-day and between-day precision. The method was applied to a small scale study of quercetin pharmacokinetics and quercetin was shown to be absorbed from a 20 mg dose. No free quercetin was detected in plasma and no evidence of significant amounts of quercetin glycosides in plasma was found.