Acridone alkaloids are well known natural products occurring solely in the family of the Rutaceae. Moreover up to now dihydrofuroacridone alkaloids have been isolated predominantly from the species Ruta graveolens (1). As exemplified by the effects of rutacridone, rutacridone epoxide, and isogravacridonchlorine, which are a part of our work on the investigation of the genotoxicity of natural compounds, these substances are of biological importance, since they all exhibit a strong mutagenic activity (2, 3). Here we give a report on the isolation and identification of a new dihydrofuroacridone named gravacridondiolacetate (1) from the roots of Ruta graveolens, which was isolated together with the known acridones gravacridontriol (2), gravacridondiol (3), rutacridone (4), rutacridone epoxide (5), and 1-hydroxy-10-methylacridin-9(10H) one (6); Xanthine oxidase (XOD) causes gout and is also responsible for oxidative damage to living tissues (1, 2). Various types of phenolics including tannins (2, 3) have been reported to inhibit this enzyme. We have now examined the effects of several galloylated flavonoids on XOD, since galloylated flavonoids have been found to show some activities similar to those of tannins (4, 5). All of the galloylated flavonoids tested in the present experiment, except for hyperin-2"-gallate (1) (from Pyrola incarnata Fisch.) (4), have been isolated from the following new plant sources whose medicinal usages are given in references (6—8): astragalin-2"-gallate (2) and isoquercitrin-2"-gallate (3) from Euphorbia maculata L., astragalin-6"-gallate (4) from Rhus sylvestris Sieb. et Zucc., and quercitrin-2"-gallate (5) from Koelreuteria paniculata Laxmann. These galloylated flavonoids showed inhibitory effects with IC50 values 1.9—3.6 x 10^-5 M, in spite of a report that the inhibitory activity of 3 was very weak (9). The inhibitory effects of 1—5 observed in the present study are markedly stronger than the inhibitory effects of the corresponding flavonol glycosides lacking a galloyl group (1 > hyperin; 2, 4> astragalin; 3 > isoquercitrin; 5 > quercitrin). It is observed that the differences in the constituent sugars among the galloylated flavonoids noticeably affect their inhibitory activity (1 <3 < 5), and the location of the galloyl group on the sugar moiety also influences it (2 <4). These findings suggest that the spatial correlation of the aglycone and the galloyl group in each compound caused the differences in the strength of the inhibitory activity. Kinetic analysis using Lineweaver-Burk plots (Fig. 1) indicated that 5, the most effective inhibitor in the present experiment, inhibited XOD non-competitively at the concentration of 1.9 x 10^-5 M