We characterized the in vivo mechanical properties of segments of upper descending thoracic aorta (UDTA) in terms of volume distensibility, which was derived from measurements of pulsatile intravascular pressure, inner wall radius, and length changes. Data for this analysis were obtained from six dogs at rest and during moderate treadmill exercise (8% grade, 4.5 miles/hr). Volume distensibility reflects the regional rheological properties of the UDTA at in vivo states. It was shown to be the sum of circumferential extensibility, longitudinal extensibility, and higher-order extensibilities. Circumferential extensibility and longitudinal extensibility are linear expressions of vessel kinematic changes which represent percent volume changes per pulse pressure and are due to circumferential and longitudinal dimensional changes alone. The higher-order extensibilities (second and third order), however, account for the coupling effect, which is the percentage volume change per millimeter mercury pulse pressure due to the interactions among radial, circumferential, and axial dimensional changes. The volume distensibility of the UDTA during exercise was significantly less than that at rest (0.67 +/- 0.12 vs. 0.91 +/- 0.11% V/mm Hg pressure). This was the result of a significant decrease in circumferential extensibility and higher-order extensibility in response to exercise with no change in longitudinal extensibility. The higher order extensibilities were also important since the volume distensibility of the UDTA was underestimated by 10% for both rest and exercise when they were ignored. We also evaluated radial extensibility by using pressure and wall thickness data and showed that this variable did not change in response to exercise.