The effects of the volatile oil of Rosmarinus officinalis leaves on the tracheal smooth muscle of rabbit and guinea pig were tested in vitro using isolated tracheal strips. The volatile oil of R. officinalis leaves inhibited the contractions of rabbit tracheal smooth muscle induced by acetylcholine stimulation and the contractions of guinea pig tracheal smooth muscle induced by histamine stimulation. Also, the volatile oil inhibited the contractions of rabbit and guinea pig tracheal smooth muscle induced by high potassium (K+) solution. This inhibition was dose-dependent and reversible. Furthermore, the volatile oil inhibited the contractions of rabbit and guinea pig tracheal smooth muscle induced by acetylcholine and histamine stimulation, respectively, in Ca(2+)-free solution. These data suggest that the volatile oil of R. officinalis leaves has a calcium antagonistic property.