This study was undertaken to assess the effects of eugenol (EUG) on tracheal muscle (TM) and the putative mechanisms underlying these effects. Cumulatively increasing concentrations (1-1000 μM) of EUG did not affect the resting tonus of TM. However, EUG (1-2000 μM) reduced the contractions induced by electrical field stimulation (IC50 = 842.3 ± 52.7 μM), an effect that was unaltered by either 10 μM montelukast (IC50 = 816.1 ± 70.1 μM) or 2 μM indomethacin (IC50 = 693.1 ± 170.8 μM). EUG also completely relaxed the sustained contractile responses to 80 mM K+ (IC50 = 597.3 ± 60.6 μM) and 1 μM carbamoylcholine (IC50 = 571.3 ± 148.8 μM), an effect that was unaltered by indomethacin (2 μM). Under Ca2+-free conditions, EUG reduced the ACh-induced contractions (IC50 = 703.4 ± 256.1 μM), the CaCl2-induced contractions in preparations pretreated with 60 μM ACh in the presence of nifedipine, and the Ba2+-induced contractions in preparations depolarized with K+. In tracheal preparations maintained in Ca2+-containing solution, EUG (300-2000 μM) relaxed the contractile response to phorbol dibutyrate (1 μM), an activator of protein kinase C. It is concluded that in TM, EUG induces a myogenic antispasmodic effect (not modulated by arachidonic acid derivatives) either through various mechanisms almost with the same pharmacological potency or via an action on a step common to all of them. These mechanisms seem to include blockade of voltage- and receptor-operated Ca2+ channels, IP3-induced Ca2+ release from sarcoplasmic reticulum and reduction of the sensitivity of contractile proteins to Ca2+.