The factors responsible for collateral arterial growth after major artery occlusion remain obscure, despite their importance for tissue survival. An increase in endothelial cell labelling with tritiated thymidine, as an index of collateral arterial growth, occurs early after renal artery occlusion. Our working premise was that an increase in endothelial cell labelling would occur simultaneously throughout the length of the collateral arteries if biophysical factors related to blood flow were the responsible mechanism, because blood flow must be increased simultaneously throughout the length of the small, preformed collateral arterial vessels. On the other hand, if the information spread from the ischemic zone, one would anticipate centripetal spread of the endothelial cell hyperplasia in a retrograde direction from the ischemic zone. With the periureteric collateral arterial supply as the model, we performed serial studies of tritiated thymidine labelling following renal artery stenosis in the rat. As anticipated, endothelial cell labelling rose sharply within 24 to 48 hours, first evident in the area immediately adjacent to the renal hilum. Thereafter, a progressive, time-related centripetal gradient in endothelial cell tritiated thymidine labelling occurred (p <0.01). These findings indicate that the factors responsible for endothelial cell hyperplasia are less related to blood flow in the lumen than to downstream, ischemic events. Although the mechanism responsible for the centripetal spread remains speculative, the communication system is likely to involve cell-to-cell contact in the vessel wall.