A series of 14 vein-to-artery grafts, 1 mm in diameter and 5 mm long, were inserted microsurgically into iliac arteries of rats. They were analysed histologically 8-18 months later and compared with control iliac arteries in the same rats. Neo-intimal hyperplasia developed and was measured in all grafts but the values did not significantly exceed the equivalent intimal plus medial thicknesses of control arteries. Vasa vasorum developed and were quantitated as the number of vessels per mm2 of neo-intima, but also did not differ significantly from control values. The density of sympathetic innervation was quantitated using fluorescent catecholamines. There was an overall significant increase in the long-term graft innervation compared with control arteries. These results show that such small vein grafts adapt and function very effectively, in a manner remarkably similar to the artery they replace, for long periods of time.