Effects of ulinastatin (a Kunitz-type proteinase inhibitor) administration was examined in a model of acute ischemic renal injury induced by bilateral renal artery occlusion in rats. Compared with rats administered vehicle, rats administered ulinastatin (150,000 U/kg body weight) intravenously 30 min preischemia had significantly lower serum creatinine and blood urea nitrogen, and much less injury evident by examination of kidney histologies over the course of 48 h postreperfusion. We conclude that ulinastatin exerts a protective effect against ischemic renal injury.