A scanning electron-microscopic study of the intimal surface of anastomosed arteries of experimental arteriovenous fistulae in 20 rabbits was conducted to determine the effect of altered haemodynamics on the arterial wall. Control arteriotomies were performed on 12 animals on the contralateral common carotid arteries. The rabbits were killed from 1 to 447 days postoperatively. Transverse intimal tears involving the internal elastic lamina developed in both proximal and distal segments of the anastomosed artery as early as 5 days postoperatively. Some were covered with thrombus and attenuated endothelial cell remnants, but all tears became endothelialised by 7 days. Endothelial cells in the floor of the tears had prominent nuclei and were smaller and more plentiful than the normal intima but, in longstanding fistulae, they resumed a more elongated spindle shape. Tears similar to those in the experimental arteries were found in most control arteriotomies, being confined to the suture zone. The results reveal the profound effect of altered haemodynamics on the arterial wall.