Seven flavonoids of Stachys chrysantha and Stachys candida have been isolated. The structures of the compounds were elucidated by spectroscopic methods, particularly highfield NMR spectroscopy. The effects of the methanol extracts of these two endemic Greek Stachys sp. and their main flavonoids were examined on arachidonic acid (AA) metabolism in the cellular system (mouse peritoneal macrophages and human platelets). Their cytotoxicity on cells was also investigated. Most samples assayed did not exhibit any significant effect on prostaglandin E₂ (PGE₂)-release from calcium ionophore-stimulated mouse peritoneal macrophages. Only chrysoeriol-7-O-β-D-(3"-E-p-coumaroyl)-glucopyranoside, at the highest non-cytotoxic dose (50 μM), inhibited the release of PGE₂, but this effect is not statistically significant. The release of leukotriene C₄ (LTC₄) by mouse peritoneal macrophages stimulated with calcium ionophore was inhibited by a crude extract of S. chrysantha, with an IC₅₀ value of 34.3 μg/ml. Xanthomicrol (IC₅₀=29.2 μM) and chrysoeriol-7-O-β-D-(3"-E-p-coumaroyl)-glucopyranoside (IC₅₀=11.1 μM) also inhibited the release of LTC₄, althouth it showed less potency than the reference compound nordihydroguaiaretic acid (NDGA) (IC₅₀=2 μM). However, most samples assayed showed a significant effect on thromboxane B₂ (TXB₂)-release from calcium ionophore-stimulated human platelets, with inhibition percentages slightly lower than the reefrence drug ibuprofen (IC₅₀=7 μM). The IC₅₀ values are : crude extract of S. candida 23.3 μg/ml; crude extract of S. chrysantha 23.1 μg/ml; xanthomicrol 28.8 μM; calcycopterin 2.66 μM and chrysoeriol-7-O-β-D-(3"-E-p-coumaroyl)-glucopyranoside 8.8 μM. Our results indicate that the selective inhibition of TX-synthase enzyme may be the primary target of action of most of these samples, and one of the mechanisms through which thus exert their antiinflammatory effects.