The methanolic extract from a Chinese herbal medicine, the rhizome of Alisma orientale, was found to exhibit inhibitory activity of nitric oxide (NO) production in lipopolysaccharide (LPS)activated macrophages. Novel triterpenes, alismaketones-B 23-acetate and -C 23-acetate, were isolated from the active extract together with eight sesquiterpenes and eighteen protostane-type triterpenes. The absolute stereostructures of new triterpenes were characterized on the basis of chemical and physicochemical evidence, which included the chemical correlations with known triterpenes. The guaiane-type sesquiterpenes (alismol, orientalols A and C) and protostane- and seco-protostane-types triterpenes (alisols C monoacetate, E-23-acetate, F, H, I, L-23-acetate, and M-23-acetate, alismaketones-B 23-acetate and -C 23-acetate, alismalactone 23-acetate, and 3-methylalismalactone 23-acetate) inhibited LPS-induced NO production (IC50 = 8.4-68 microM). Other triterpenes (alisols A, A monoacetate, B, B monoacetate, E, G, K-23-acetate, and N-23-acetate and 11-deoxyalisol B) also showed the potent inhibitory activity, but they showed cytotoxic effects more than 30 microM (MTT assay). In addition, alismol and alisol F were found to suppress iNOS induction.