Rabbit aortic strips were subjected to a load of 1 gm of tension and mounted in a bath medium at 37℃, aerated with 50% CO_2 and 95% O_2. Adrenaline (10^<-4> M) was added to the bath medium to contract these strips. In the aortic strips (the aortic arch, and the thoracic aorta), activation of glycogen phosphorylase was observed during contraction. The activity of glycogen phosphorylase a in the contracting aortic arch was increased by 31% of the control activity, while the increase in the enzymatic activity in the contracting thoracic aorta was 28%. However, the activity of the abdominal aorta was unchanged during contraction. From these findings,it appears that glycogenolysis increases in the contracting aortic arch and thoracic aorta.