Intravascular devices are provided for forming a vascular graft by axially distending a blood vessel to induce growth. These devices advantageously can be implanted via a catheter, thereby eliminating the need for a more invasive implantation procedure when the stretching is to be done in vivo. The implantable medical device for distending a blood vessel to induce axial growth of the blood vessel includes an intravascular stretching mechanism securable to an interior luminal surface of a blood vessel in vivo, and a means for operating the intravascular stretching mechanism in vivo to cause the vessel to stretch and grow axially. The stretching mechanism can include a pair of wires or stents that engage the blood vessel wall, and components of the stretching mechanism may include a shape memory material.