The durations of α-glucosidase inhibitory activity of various sugars administered to unrestrained, unanesthetized rats via portal cannulae were examined. The measured duration were the times for which elevation of portal glucose levels resulting from continuous intragastric infusion of maltose or sucrose were suppressed by the sugars. D-Xylose (0.24g/kg), L-arabinose (0.24g/kg) and D-glucurono-6, 3-lactone (0.24g/kg), as well as acarbose (0.024g/kg) and voglibose (0.00024g/kg), α-glucosidase inhibitors, suppressed the elevation of portal glucose levels. The durations of the inhibitory effects of D-xylose, L-arabinose, D-glucurono-6, 3-lactone, acarbose and voglibose were 150min, 60min, 90min, 120min and >170min, respectively. These findings indicate the importance of measuring the duration of the inhibitory effect on α-glucosidase when studying α-glucosidase inhibitors.