As a part of our continuing search for biologically active substances from medicinal plants in Paraguay, we have examined inhibitory activities of 70% EtOH extracts of about 60 native crude drugs against xanthine oxidase (XO, EC 1.2.3.2). We report here that the extract of the aerial parts of Paraguayan "Molle-i" (Schinus terebinthifolius Raddi, Anacardiaceae) showed significant inhibitory activity (79.8% at 50 μg/ml) against cow's milk xanthine oxidase. Flavonoids (1-5), hydroxychalcones (1), coumarins (1), 2,6-dihydroxyadenine (6), and xanthones (7) have previously been reported as naturally occurring inhibitors of XO. An inhibitor of XO could potentially be useful as a therapeutic agent for hyperuricemia that causes gout (8), renal stones (8,9), or ischemic myocardium (10).Bioassay-directed fractionation of the extract resulted in the isolation of an active substance as a colorless, amorphous powder. The compound was positive in the FeCl3 reaction and showed absorption bands at 280 nm and 3400, 1720, and 1605 cm-1 in the uv and ir spectra, respectively. The 1H-nmr spectrum indicated the presence of oxymethylene protons (δ 4.42, dd, J=4.4, 11.8 Hz; 4.54, d, J= 11.8 Hz), of five oxygenated methine protons (δ 4.54, m; 5.61, dd, J=9.8, 8.3 Hz; 5.66, t, J=9.8 Hz; 6.00, t, J=9.8 Hz; 6.33, d, J= 8.3 Hz), and of ten aromatic protons (δ 6.97, 7.01, 7.05, 7.11, 7.18, each 2H, s), which were characteristic of pentagalloylglucose (11). The compound was identified as 1,2,3,4,6-penta-O-galloyl-β-D-glucose by direct comparison (tlc, ir, 1H nmr, [α]D) with an authentic sample.The inhibitory activity against XO of pentagalloylglucose was then compared with that of allopurinol, a well known XO inhibitor (12). As shown in Table 1, pentagalloylglucose exhibited the same level of activity as allopurinol. Kinetic analysis of the reaction of XO with pentagalloylglucose by Lineweaver-Burk plots revealed that it inhibited XO noncompetitively at the concentration of 2.12 × 10^-5 M. Inasmuch as pentagalloylglucose was reported to inhibit sialidase, hyaluronidase, alkaline phosphatase, and cholesterol oxidase (13), the inhibitory activity might be due to its tannic character. On the other hand, pentagalloylglucose was found to exert an antiherpetic (antiviral) effect (14). This is the first reported isolation of 1,2,3,4,6-penta-O-galloyl-β-D-glucose from S. terebinthifolius.