We have investigated angiographically and histologically the vascular response to implantation of intravascular stents in normal puppies. Eight balloon-expandable stents were placed in six mongrel puppies (weight 6.4–10.5 kg) under anesthesia. Four stents were successfully placed in the pulmonary arteries and three in the aorta. In one case, the strut of the stent was hooked by a leaflet of the tricuspid valve at the time of placement and eventually had to be implanted in the inferior caval vein. Two puppies died, one from bleeding from the femoral artery and the other from thrombotic obstruction at the superior mesenteric artery, both after successful implantation. The former was due to repeat arterial puncture and the latter wasdue to failure to use aspirin. Angiographic studies showed satisfactory opacities of the branch arteries in all but one puppy. The diameter of the arteries prior to implantation was 7.5±1.5 mm. The diameters at the middle of the stents immediately after implantation and during the period of follow-up were 6.9±1.7mm and 6.9±2.1 mm, respectively. Histologic studies revealed that neogenic endothelial layers of 30 to 40 pm in thickness had formed over the struts of the stent without formation of thrombusthree weeks after implantation. Scanning electron microscopy revealed that the endothelial cells of the neoendothelial layer had almost identical characteristics to those of normal arteries. These results indicate that clinical application of this stent is feasible but that use of anticoagulants is mandatory during and after implantation in the treatment of selected patients with aortic coarctation or pulmonary arterial stenosis. Relative progression of stenosis may occur, however, and the inability for re-expansion is a major disadvantage of this stent.