Iliolumbar vein to iliac artery grafts were placed in 40 rats by microsurgical technique. Groups of animals were perfused with fixative at eight intervals between one and 20 weeks after operation, and sections of the graft and control arteries (the opposite iliacs) were analyzed microscopically. The revascularization of the graft by capillaries commenced within the first postoperative week. The level of vascularity (capillaries per cross-sectional mm2) increased during the first four weeks, maintained a constant level and declined after week 16. The grafts of the 17--20 week group were substantially less vascular than the earlier groups. Intimal thickening commenced at three to four weeks after operation, i.e. during the period of increasing graft vascularity. The mean intimal proportion of the graft was 14% at four to five weeks and at 17--20 weeks was 35% of the cross-sectional area of the graft wall. However, the actual thickness of the intima did not increase significantly with time, rather the whole graft wall tended to become thinner. At 17--20 weeks grafts which showed a high degree of intimal thickening had significantly fewer capillaries within their walls. Quantitative evidence is presented to suggest that the continued growth of the graft intima may not be supported by a similar increase in the number of vasa vasorum. Therefore, it is suggested that the reduced level of vascularity in grafts with hyperplastic intimae may form an ischemic basis for degenerative changes which are known to take place in some long-term grafts.