The effects of the essential oil of Eucalyptus tereticornis Sm. (EOET) on guinea-pig tracheal smooth muscle were investigated. EOET (10 - 1000 microg/mL) relaxed the tracheal basal tonus with an EC (50) value of 125.3 [52.2 - 300.9] microg/mL. Its maximal relaxation (40 +/- 6 %) was significantly