Rhizomes of butterbur, Petasites hybridus L. (Asteraceae), have been used since ancient times for the treatment of inflammatory diseases. In the present study, the effects of lipophilic extracts from rhizomes of Petasites hybridus on the formation and release of prostaglandin E2 were investigated. The extracts had different contents of petasin and isopetasin: A: 2.1 % and 0.4 %, B: 0.2 % and 0.1 %, C: 12.1 % and 6.1 % and D: 21.9 % and 9.4 %, respectively. Direct inhibition of cyclooxygenase (COX) -1 and -2 isoenzymes and inhibition of the expression of COX-2 and p42/44 MAP kinase in rat primary microglial cells were tested. All extracts were found to be only weak direct inhibitors of COX-1 (IC50> 400 microg/mL). However, most extracts revealed a strong inhibitory activity against the inducible isoform COX-2 ( A: IC50=30.4 microg/mL; B: IC50=60.6 microg/mL; C: IC50=22.6 microg/mL; D: IC50=20.0 microg/mL). This activity was not correlated to the content of petasin and isopetasin. Pure petasin and isopetasin neither inhibited COX-1 nor COX-2 (IC50 > 400 microM for both compounds and enzymes). Petasites extracts dose-dependently inhibited LPS-induced and thus COX-2-mediated PGE2 release in primary rat microglial cells (A: IC50= 2.4 microg/mL; C: IC50=5.8 microg/mL and D: IC50=4.6 microg/mL). Also this effect was independent from the petasin and isopetasin content. COX-2 synthesis in microglia was totally blocked with 5 microg/mL of C whereas COX-1 synthesis was not influenced. C and D did not affect the LPS-induced activation of p38 MAPK and IkappaBalpha, but they prevented the LPS-induced activation of p42/44 MAPK. Therefore, these Petasites hybridus extracts can be regarded as natural selective inhibitors of COX-2 and its expression, an effect which is independent from the petasin content.