Inner medullary blood flow has been found to remain nearly unchanged during postischemic renal failure, despite a severely disturbed perfusion in the outer medulla. In order to elucidate this discrepancy, rats were subjected to 1 h left renal artery occlusion and 1 h reflow. The blood plasma was then labeled by dye-conjugated globulin for 1 min. The in vivo indicator distribution was histologically analyzed, especially in the medullary vascular bundles. The filling there was observed along a section plane positioned through the inner stripe along the long and the short axis of the organ. Vessels centrally located within the bundles were more labeled than those in the periphery of the bundles. In addition, the degree of filling on the whole was almost twice as high in the tissue near the renal sinus as in the central area. At the same time, filling defects in the renal papilla were restricted mostly to the center of that tissue. The observations support the assumption that differences in blood flow obstruction exist within each bundle. They also show that perfusion defects of the outer medulla occur more readily in the center of the organ, thus allowing blood to enter the inner medulla via vascular bundles located marginally, adjacent to the renal sinus of the kidney.