This study elucidated possible mechanisms for the different anti-inflammatory potencies exhibited by the water extracts of Eucommia ulmoides bark and Plantago asiatica seeds, which contain various iridoids. Water extracts of both herbal materials were tested in vitro with a battery of assay models: lipopolysaccharide-induced thromboxane B(2) for cyclooxygenase-1 (COX-1), prostaglandin E(2) for cyclooxygenase-2 (COX-2), the translocation of nuclear factor-kappaB (NF-kappaB), and tumor necrosis factor alpha (TNFalpha) and nitric oxide (NO) production in RAW 264.7 cells. The contents of the iridoid glycosides, aucubin (AU), catalpol (CA), and geniposide (GE), were quantified by high-performance liquid chromatography (HPLC). Neither E. ulmoides nor P. asiatica suppressed the COX-1 enzyme. P. asiatica significantly inhibited COX-2 (concentration required for 50% inhibition [IC(50)] = 8.61 (microg/mL), TNFalpha (IC(50) = 9.63 (microg/mL), and NO (IC(50) = 8.65 (microg/mL) production. P. asiatica blocked the translocation of NF-kappaB from the cytosol to the nucleus. E. ulmoides suppressed only the COX-2 enzyme (IC(50) = 9.92 (microg/mL). The results of the HPLC analysis revealed that P. asiatica contained three iridoid glycosides, AU, CA, and GE. E. ulmoides contained CA and GE, but no AU was detected. The difference in the anti-inflammatory potencies of E. ulmoides and P. asiatica appears to be dependent on the presence of AU. Considering the IC(50) values, both herbal extracts exhibit modest and less potent anti-inflammatory activities than common synthetic non-steroidal anti-inflammatory drugs.