The effects of eugenol on the excitation-contraction coupling of the toad (Bufo paracnemis) sartorius were studied. Eugenol (0.1-2.5 mM) blocked the contracture induced by 80 mM K+. At 2.5 mM, it decreased the depolarization, induced by 80 mM K+, by 14 mV. This is not sufficient to explain the complete blockade of the potassium contracture. At higher concentrations (3-12 mM), eugenol induced contractures, without causing a depolarization of the membrane. Contractures were blocked by procaine and potentiated by caffeine and by cooling of the muscle. For concentrations of eugenol in the upper range of the dose-response curve (10 mM), the contracture was independent of the Ca++ concentration in the medium. At lower concentrations (5 mM), the contractures induced by eugenol were potentiated by divalent cations, in the order: Mg++ < Ba++ < Zn++ < Co++ < Ca++ < Mn++. We infer that the eugenol-induced contractures are caused, primarily, by a release of Ca++ from the sarcoplasmic reticulum and, secondarily, by a mechanism related to the presence of some divalent cations in the extracellular space.