Bioassay-guided fractionation of a CH2Cl2-soluble fraction of the stems of Catalpa ovata led to isolation of a new naphthoquinone, 4-hydroxy-2-(2-methoxy-3-hydroxy-3-methyl-but-1-enyl)-4-hydro-1H-naphthalen-1-one (10), together with nine known compounds, catalponol (1), catalponone (2), catalpalactone (3), alpha-lapachone (4), 9-hydroxy-alpha-lapachone (5), 4,9-dihydroxy-alpha-lapachone (6), 9-methoxy-alpha-lapachone (7), 4-oxo-alpha-lapachone (8), and 9-methoxy-4-oxo-alpha-lapachone (9). The structures were elucidated on the basis of spectroscopic analyses. The inhibitory effects of these isolates on lipopolysaccharide-induced NO synthesis in RAW 264.7 cells were evaluated. Among them, catapalactone (3), 9-hydroxy-alpha-lapachone (5) and 4,9-dihydroxy-alpha-lapachone (6) exhibited potent inhibitory effects, with IC(50) values of 9.80, 4.64 and 2.73 microM, respectively.