Schmeda-Hirschmann, G. Departamento de Ciencias Biologicas, Universidad de Talca, Casilla 747, Talca, Chile.The methanol extract of the rhizomes of Scirpus californicus (Cyperaceae) inhibited the enzyme xanthine oxidase (XO) in vitro. Bioassay-guided isolation led to piceatannol, scirpusin A and scirpusin B as the main XO inhibitors of S. californicus. Piceatannol, scirpusin A and B were mixed-type inhibitors of the XO with Ki values of 1.1×10-4, 6×10-4 and 5×10-5 M, respectively. The extract and above mentioned compounds were marginally active as hypotensors when tested in normotensive rats at 2.5 mg/kg and were inactive towards the DNA binding assay in vitro