Spirodela polyrhixa Schleid has been used in folk medicine to treat inflammatory diseases. Since nitric oxide (NO) is one of the major inflammatory parameters, we studied the effect of aqueous extracts of Spirodela polyrhixa (AESP) on NO production in lipopolysaccharide (LPS)-stimulated mouse peritoneal macrophages. AESP inhibited the secretion of NO in macrophages, without affecting cell viability. The protein level of inducible nitric oxide synthase (iNOS) in peritoneal macrophages was also decreased by AESP. Transient transfection assay of reporter plasmid and gel shift assay indicated that AESP blocked the activation of nuclear factor-kappa B (NF-kappa B), which was considered to be a potential transcription factor for iNOS expression. AESP also blocked the phosphorylation and degradation of inhibitory protein I kappa B-alpha (I kappa B-alpha). These results suggest that AESP could exert its anti-inflammatory actions by suppressing the synthesis of NO through inhibition of NF-kappa B activity.