Instantaneous pressure-radius-length relationship was studied in the ascending aorta of ten living dogs. Results indicate: 1) longitudinal and circumferential strains in the ascending aorta are large as compared to other segments of the aorta, and 2) it is suggested that the intravascular pressure may not be the only radially distending stress since other forces due to contraction of the heart seem to play a role in this local area. Note:With the Technical Assistance of Joseph M. Pearce mechanical properties ascending aorta living dog; stress-strain relation ascending aorta living dog Submitted on September 27, 1963