The extracts of Oenanthe javanica were evaluated for their effects on the expression of cyclooxygenase-2 (COX-2), which is mediated by the translocation of the p65 subunit into the nucleus. Fractions of ethyl acetate and chloroform from 80% ethanol extracts of O. javanica exhibited inhibitory effects on the secretion of tumor necrosis factor-${alpha}$ (TNF-${alpha}$) from lipopolysaccharide (LPS)-stimulated peritoneal macrophages; however, the aqueous- and hexane-fractions showed no significant effect. The ethyl acetate- and chloroform-fractions also reduced the COX-2 enzyme levels after 24-hr treatment. RT-PCR showed that the mRNA levels of COX-2 decreased following treatment with these fractions, suggesting that COX-2 expression is transcriptionally regulated by these extracts. We examined the effects of the chloroform- and ethyl acetate-fractions on the cytosolic activation of nuclear factor-${kappa}B$ ($NF-{kappa}B$, p65 subunit) and on the degradation of inhibitor-${kappa}B{alpha}$ ($I-{kappa}B{alpha}$) in order to determine the mechanism of COX-2 regulation. The LPS-stimulated activation of the p65 subunit was significantly blocked upon the addition of $50;{mu}g/mL$ of these fractions, and the cytosolic $I-{kappa}B{alpha}$ degradation process was simultaneously inhibited. These findings suggest that the inhibition of COX-2 expression by the ethyl acetate-and chloroform-fractions may result from the inhibition of p65 translocation by blocking the degradation of $I-{kappa}B{alpha}$; this may be the mechanistic basis for the anti-inflammatory effects of O. javanica.