Neointimal hyperplasia is a common finding after trauma to blood vessels and also as a primary change in atherosclerosis. In this study we have developed a simple model, using microsurgical techniques, for the initiation of neointimal hyperplasia in the rat In 24 Wistar rats, a 2 mm-diameter arteriotomy in the aorta was repaired with a ''patch'' of iliolumbar vein, using eight evenly spaced 10-0 Ethilon sutures. The patch overlapped the edges of the arteriotomy, and the sutures fastened the patch to the subjacent aorta. At 2, 6 and 12 weeks after surgery, the venous patch grafts and segments of the adjacent aortae of eight rats were removed. One-half of the specimens were analyzed by scanning electron microscopy and the other one-half by light microscopy. All patch grafts were surgically successful. Endothelial cells regenerated to cover the patch within 2 weeks of insertion. By 6 weeks after surgery, neointimal hyperplasia, consisting predominantly of smooth muscle, had developed in all patches to a thickness that was not significantly different from that of the adjacent aorta. These findings are consistent with data from other more complex experimental models of neointimal hyperplasia in the rat. We consider that this venous patch technique is a simple but effective model for the initiation of neointimal hyperplasia in the rat and may easily be used to study the experimental effects of various injurious or therapeutic agents on neointimal hyperplasia.