Segments of the canine ascending aorta, upper descending thoracic aorta, and middle descending thoracic aorta were instrumented with ultrasonic dimension gauges and a cathetertip manometer simultaneously to measure changes in segment diameter, length, and intravascular pressure. Volume distensibility (EV) was calculated as the sum of circumferential extensibility (EC), longitudinal extensibility (EL), and high order extensibilities (EK) for each segment. The EC and EL were linear expressions that represented percentage volume changes per mmHg pulse pressure due to circumferential and longitudinal dimensional changes. The high order extensibilities (second and third order) accounted for the percentage volume changes per mmHg pulse pressure due to the interactions between circumferential and longitudinal dimensional changes. Mean(SEM) EV values from six dogs were 1.62(0.31), 0.84(0.08), and 0.62(0.08)% delta V/mmHg delta P for the ascending aorta, upper descending thoracic aorta, and middle descending thoracic aorta segments respectively. The EV, EL, and EK of the ascending aorta segment were significantly greater than those of the upper descending thoracic aorta and middle descending thoracic aorta segments, whereas EC was significantly less in the ascending aorta than in both the upper descending thoracic aorta and middle descending thoracic aorta segments. It is concluded that there are regional differences in aortic distensibility and its components in vivo. Longitudinal wall motion is an important determinant of these aortic mechanical properties.