The effect of the nature of the sugar moiety on quercetin absorption has been investigated in rats. Four groups of rats received an experimental meal containing 20 mg of quercetin equivalents, supplied as quercetin, quercetin 3-O-beta-glucoside, quercetin 3-O-beta-rhamnoside or rutin. Four hours after the meal, the metabolites identified in hydrolysed plasma were identical in all groups (3'- and 4'-methylquercetin). However, the total concentration of metabolites was markedly different: 11.2+/-1.8, 2.5+/-2.0 and 33.2+/-3.5 microM for the quercetin, rutin, and quercetin 3-glucoside meals respectively. After quercetin 3-rhamnoside consumption, we failed to detect any metabolites in the plasma. These data suggest that the 3-O-glucosylation improves the absorption of quercetin in the small intestine, whereas the binding of a rhamnose to the aglycone markedly depresses it. Additional experiments have shown that the higher plasma levels measured after quercetin 3-glucoside meal compared to the quercetin meal were maintained throughout the 24-hour period following the meal. Using a multi-electrode coulometric detection, together with suitable chromatographic conditions, we were able to distinguish between the conjugated and the glycosylated forms. Thus, we clearly showed the absence of quercetin 3-O-beta-glucoside in the plasma from rats fed a diet containing this glucoside. This result suggests that quercetin 3-O-beta-glucoside is hydrolysed before or during its intestinal absorption.