We estimated glycogen phosphorylase activities in the aorta and iliac artery in rabbits and the following results were obtained :1) In parallel with the amounts of protein and glycogen, the levels of total glycogen phosphorylase and glycogen phosphorylase a decreased from the arch of aorta to the iliac artery. 2)The level of glycogen phosphorylase b was higher in the arch of aorta than in the thoracic and abdominal aortae and the iliac artery.