Thyme extract is known to posses spasmolytic effects on isolated trachea and to increase ciliary activity. A small part of this effect is mediated via beta (2) receptors, but other receptors are supposed to be involved. Endothelin has a pathophysiological impact in asthma with respect to hyper-reagibility and contracting isolated trachea smooth muscles and was, therefore, investigated. Thyme extract inhibited endothelin-induced contraction. Emax decreased depending on the concentration of thyme extract indicating a non-competitive inhibition. Bosentan, an endothelin receptor antagonist (positive control), acted as a competitive inhibitor. The interaction/combination of thyme extract and bosentan was additive but not supra-additive indicating an interaction with the endothelin system. Thymol (one of the major compounds of thyme extract) did not interact with the endothelin system. It is concluded that thyme extract may help in diseases related to endothelin hyper-reagibility of the bronchus system such as asthma and COPD (chronic obstructive pulmonary disease), although thymol is not involved in this effect.