We present results of a pilot study of ex vivo and in vivo acoustic radiation force impulse (ARFI) imaging demonstrating measurements of the mechanical properties of the medial and adventitial layers in the carotid, and popliteal arteries. The data was obtained on a commercial scanner, providing co-registered B-mode and Color Doppler images. A real-time ARFI imaging system capable of achieving frame rates of 0.75 Hz was developed. The 2-D and 1-D images of the tissue's response to brief and localized applications of radiation force show good correlation with B-mode and pathology based characterization of vessel geometry and plaque stiffness. Measurements of arterial response during both systole and diastole are presented. We address implementation issues and discuss the potential applications of this new vascular imaging method.