Abstract: Cinobufagin, one of the active principles extracted from toad venom,was studied on the isolated vas deferens of the guinea pig. The preparation was suspended in a bath containing 10 ml Krebs solution. Isometrictension of vas deferens was recorded on polygraph with a force displacement transducer. Cinobufagin caused a long- lasting contraction of the vas deferens of the guinea pig-at concentrations of 15 ～50 μmol/L. The contraction was inhibited following reserpini-zation and cold storage treatment.The cinobufagin induced contraction was partlyblocked by pretreatment with phentolamine and verapamil. These results suggest that the cinobufagin induced contraction of vas deferens mightbe related to an action which promotes release of NA from the adrenergic nerve terminals.