"Paraparai mi," Phyllanthus niruri L. (Euphorbiaceae), has traditionally been used for the treatment of urolitic disease and as a diuretic in Paraguay. In the screening tests for biological activity of about 60 Paraguayan medicinal plants, however, the 70% EtOH extract of "paraparai mi" was found to have inhibitory activity against angiotensin-converting enzyme (ACE). In continuation of our studies, the n-BuOH extract of the medicinal plant was found to have higher activity than the EtOH extract. ACE is a dipeptidyl carboxypeptidase that plays an important role in blood pressure regulation by catalyzing an important reaction. Several inhibitors of ACE from plant sources have been reported. Previously, Inokuchi and coworkers investigated ACE inhibitors extracted from Chinese crude drugs (2,3) and reported the inhibitors from the seeds of Areca catechu (4). We also reported inhibitory activity of crude extracts from natural products on ACE (5). Recently, Uchida et al. isolated some condensed tannins from "rhei rhizoma" as ACE inhibitors (6), and Kameda et al. reported (+)-catechin as the inhibitor from Quercus stenophylla (7). P. niruri has already afforded alkaloids (8), lignans (9-12), flavonoids (13-16), lup-20(29)en-3β-ol (17), phthalic acid ester (18), fatty acid (19), and vitamin C (20). In pharmacological studies of the plant, diuretic action (21) and inhibition of induced liver injury (22) have been reported. In this paper we report the substances that inhibit ACE activity from this medicinal plant. The n-BuOH extract, which was the active extract, was applied to a column of Sephadex LH-20, which was eluted with H2O followed by 50% MeOH, MeOH, and Me2CO. Based on the results of the activities of the extracts and eluates, it seemed that the active component in the EtOH extract was extracted with n-BuOH and then concentrated in the 50% MeOH eluate (Table 1). The 50% MeOH eluate was further chromatographed over a Sephadex LH-20 column and was eluted with 70% MeOH to afford three compounds. They were identified as ellagic acid, geraniin, and gallic acid by direct comparison with authentic samples. The isolated compounds and corilagin, a hydrolysis product from geraniin, were tested for inhibitory activity against ACE together with captopril. The 50% inhibitory concentration (IC50) values of the test compounds are shown in Table 1. It was found that the IC50 for ACE activity of geraniin, the most active of the isolated compounds, was 4.0×10^-5 mol/liter. In order to determine the type of inhibition, the kinetics of inhibition of ACE by geraniin were plotted according to Lineweaver and Burk. Geraniin produced a noncompetitive inhibition pattern at the concentration of IC50. Ondetti et al. found that ACE was a zinc-containing metalloprotein (23). On the other hand, Okuda et al. reported that geraniin interacted with the heavy metal ions (24). The present study found that geraniin was a noncompetitive inhibitor for the enzyme. This result suggested that geraniin might interact with the zinc atom in ACE.