In order to clarify the mechanism involved in the antiinflammatory activity of atractylenolide I and atractylenolide III from the rhizomes of Atractylodes macrocephala Koidz, their effects on tumor necrosis factor-α (TNF-α) and nitric oxide (NO) production in peritoneal macrophages were examined. Atractylenolide I and atractylenolide III decreased the TNF-α level in LPS-stimulated peritoneal macrophages in a dose-dependent manner, their IC50 values were 23.1 µm and 56.3 µm, respectively. RT-PCR analysis indicated that they inhibited TNF-α mRNA expression. Furthermore, they inhibited NO production in LPS-activated peritoneal macrophages, the IC50 value of atractylenolide I was 41.0 µm, and the inhibition ratio of 100 µm of atractylenolide III was 45.1% ± 6.2%. The activity analysis of inducible nitric oxide synthase (iNOS) indicated that they could inhibit the activity of iNOS, their IC50 values were 67.3 µm and 76.1 µm, respectively. Western blot analysis showed that atractylenolide I and atractylenolide III attenuated LPS-induced synthesis of iNOS protein in the macrophages, in parallel. These results imply that the antiinflammatory mechanism of atractylenolide I and atractylenolide III may be explained at least in part, by the inhibition of TNF-α and NO production. Atractylenolide I showed more potent inhibition than atractylenolide III in the production of TNF-α and NO in LPS-activated peritoneal macrophages. So, atractylenolide I could be a candidate for the development of new drugs to treat inflammatory diseases accompanied by the overproduction of TNF-α and NO. Copyright © 2007 John Wiley & Sons, Ltd.