The experimental data obtained on the viscoelastic behavior of isolated aortic strips of the dog consist of: (1) the resonant or natural frequency of forced or free longitudinal vibration; (2) the logarithmic decrement. From these data, static and dynamic moduli and the time constants for the specimens are computed. Variations of these values with frequency, temperature and elongation are presented. The experimental observations are not adequately explained by a simple mechanical analogue. An alternative explanation is offered based on studies of dynamic elasticity in high polymers.