After allyl formate (AF) was administered to the rats, the existence of semicarbazide-sensitive amine oxidase (SSAO) in rat identified. 2. When the heart homogenate and plasma of AF-administered rat were pretreated with 10(-3) M clorgyline and deprenyl, the Km value for benzylamine of rat heart was same as the value of plasma. 3. The existence of SSAO in plasma of AF-administered rats were identified by IEF-gel electrophoresis. The pI values of SSAO in heart and plasma were a single peak of 5.0. 4. SSAO released from the rat heart in response to AF, although the other origins of this enzyme are unknown.