The measurement of the relationship between aortic diameter and aortic pressure is of importance in the general problem of blood flow measurements. This relationship was examined in the upper descending aorta of the anesthetized thoracotomized dog using a specially devised strain-gauge caliper and a catheter-manometer pressure recording system. It was found that the slope of the graph of mean diameter vs. mean pressure (“mean extensibility index”) was linear between mean pressures of 60–140 mm Hg, and was only slightly affected by intravenous norepinephrine. The ratio of the change in diameter to the change in pressure for a single pulse (“dynamic extensibility index”) was not significantly different from the mean extensibility index below mean pressures of about 130 mm Hg. The dynamic extensibility index was moderately decreased by norepinephrine. Note: (With the Technical Assistance of Walter Gray) Submitted on August 8, 1960