The purpose of this paper is to describe the incremental dynamic anisotropic viscoelastic properties of the middle descending thoracic aorta in ten living dogs. A segment of the middle descending thoracic aorta was isolated in situ and connected to a reservoir filled with oxygenated blood. The height of the reservoir could be adjusted to impress a known pressure on the segment. In addition, a sinusoidal fluid displacement pump was connected to this assembly to superimpose small sinusoidal pressures at various frequencies (0–5 Hz); in another phase of the experiment small sinusoidal changes in length were also imposed on the segment. The pressure, the longitudinal force, and the length of the segment were monitored continuously with specially designed transducers. From these data, the incremental elastic (E′) and the viscous (E″) moduli in the circumferential (&thetas;), longitudinal (z), and radial (γ) directions were calculated around a given state of strain in the physiological range. In general, the viscoelastic moduli were functions of the initial stretches, indicating the nonlinear nature of the arterial wall. Within the physiological range of pressure,Ez′ >E&thetas;′ >Eγ′ (P< 0.01) andEz″ >Eγ″ >E&thetas;″ (P < 0.01). These results indicated that the vessel wall was anisotropic, having its largest values of elastic and viscous moduli in the longitudinal direction. The ratio ofE′ /E′ < 0.123 at 2 Hz, indicating a small viscous component relative to its elastic counterpart. The elastic moduli increased markedly from 0–2 Hz and then settled down to a relatively constant value; this finding was consistent with previously published data. The values ofEz′ andEz″ from the in vivo experiments were higher than those from the in vitro experiments, and the difference was attributed to the vascular tethering present in the in vivo state.