Quercetin glucosides were reported to interact with the human sodium coupled glucose carrier SGLT1 and thus to inhibit glucose transport in rat jejunum epithelial cells, but whether they were also transported via SGLT1 remained controversial (Wolffram et al, J. Nutr. 132: 630, 2002 and related Letter to the editor, J. Nutr. 132:2823, 2002). We used Xenopus oocytes overexpressing SGLT1 and the two-electrode voltage clamp technique to measure transport and/or inhibition of glucose transport by flavonoids glycosylated at different positions or used as aglycones. Neither quercetin (Q), luteolin (L) or genistein nor their at different positions glycosylated (G) forms (Q-3-G, Q-4'-G, L-7-G) generated significant transport currents. However, inward current generated by the uptake of 1 mM of the SGLT1 substrate a-MDG into the oocyte was immediately and significantly reduced by the simultaneous addition of rising concentrations of Q-3-G (IC50 = 0.70 mM), Q-4'-G (IC50 = 0.09 mM), and to a lesser degree also by L-7-G. The inhibitory potency of Q-3-G remained nearly unchanged, when the attached glucose was replaced by galactose. Unexpectedly, the aglycone form of L also inhibited the transport with high affinity (IC50 = 0.18 mM). The inbibition by L was of mixed type, but selective for SGLT1, since the activity of the coexpressed proton-coupled peptide transporter PEPT1 was not reduced.