Lightly fried onions containing 275 µmoles of flavonols, principally quercetin-4′-glucoside and quercetin- 3,4′-diglucoside, were fed to healthy human volunteers with plasma and urine collected over a 24 hr period. Samples were analysed by HPLC with diode array and tandem mass spectrometric detection. Five flavonol metabolites, quercetin-3′-sulphate, quercetin-3-glucuronide, isorhamnetin-3-glucuronide, a quercetin diglucuronide and a quercetin glucuronide sulphate, were detected in plasma in quantifiable amounts with trace quantities of six additional quercetin metabolites. Sub- µM peak plasma concentrations (C_max ) of quercetin- 3′-sulphate, quercetin-3-glucuronide, isorhamnetin-3-glucuronide and quercetin diglucuronide were observed 0.6-0.8 hr after ingestion. In contrast, the C_max of quercetin glucuronide sulphate was 2.5 hr. The elimination half-lives (T_1/2 ) of quercetin-3′-sulphate, quercetin-3-glucuronide and quercetin diglucuronide were 1.71, 2.33 and 1.76 hr respectively while the T_1/2 of isorhamnetin-3-glucuronide was 5.34 hr and that of quercetin glucuronide sulphate was 4.54 hr. The profile of metabolites excreted in urine was markedly different to that of plasma with many of the major urinary components absent or present in only trace amounts in the bloodstream indicative of substantial phase II metabolism. Total urinary excretion of quercetin metabolites was 12.9 µmoles corresponding to 4.7% of intake. The bioactivity of these metabolites should be considered.