The clove ellagitannins and their related polygalloyl-glucoses inhibited maltase activity of rat intestinal alpha-glucosidases. The structure-activity relationship study of those galloylglucoses, varying the extent of galloylation on the glucose core, with the ellagitannins, indicated that an increasing number of galloyl units in the molecule lead to an increase in the inhibitory activity. Penta-O-galloyl-beta-D-glucose, with five galloyl groups showed the highest inhibitory activity. On the other hand, hexahydroxydiphenoyl units contained in the ellagitannins had little effect on the activity. After separation of maltase-glucoamylase and sucrase-isomaltase complexes from the crude mixture of the rat alpha-glucosidases, the inhibitory activities of the galloylglucose derivatives against each complex were examined. The inhibitory influence on the maltase-glucoamylase complex was more potent than on the sucrase-isomaltase complex.