BACKGROUND: Increasing degrees of medullary hyperemia induced by ischemia reperfusion injury were associated with renal dysfunction. A possible mechanism may be that ischemia causes alterations in the structure and function of vascular membranes which leads to an aggregation of red blood cells in the medullary vessel. It has been shown that heparin prevents postischemic endothelial cell dysfunction. Aim of this study was to evaluate heparin effects on renal hyperemia induced by ischemia reperfusion injury. METHOD: In this study, fifteen rabbits were randomized to either heparin treatment group(500 IU/kg IV bolus 10 minutes before renal artery occlusion, n=8) or control group(n=7). One side kidney underwent 60 minutes ischemia only by clamping renal pedicle and after that kidney tissue sample was obtained for histologic evaluation. The other side of kidney were permitted 60 minutes ischemia following 60 minutes reperfusion and after that kidney tissue sample was obtained for histologic evaluation. RESULTS: There was significant difference in the degree of congestion(2.6+/-0.2 vs 1.1+/-0.3, P<0.05) between outer medulla of control and heparin treatment group. CONCLUSION: Heparin significantly attenuated outer medullary congestion induced ischemic injury.