OBJECTIVE: To observe effects of arginine on art erial endothelium injured by ischemia-reperfusion (IR), and explore its possibl e mechanism. METHODS: Fifty-four rats were divided into 3 groups and treate d in respective ways: (1) drinking tap water as the control; (2) drinking tap wa ter containing 2.5% L-arginine; (3) drinking tap water containing 2.5% L-arginine together with intraperitoneal injection of N(G)-nitro-L-arginine methylester 5 mg/kg/d. A segment of the common carotid artery was occluded for 1 h, and then reperfused. Samples taken at different post-IR t ime from the segment were prepared for the ultrastructural and Ce-H(2)O(2) cyto chemical observation. The naked index (NI) of internal elastic lamina (IEL) was measured for comparing the endothelial injure extent and its repair process. RESULTS: Less damage of endothelial cells (EC), more platelets adhering to naked IEL and more regenerating EC were observed in Group 2. The NI values of samples taken at 1, 2, 3 d after the IR were respectively 0.92plus minus0.08, 0.88plus minus0.03 and 0.41plus minus0.02 in Group 1, and reduced to 0.52plus minus0.05, 0.19plus minus0.08 and 0.06plus minus0.01 in Grou p 2 (P<0.05-0.01). In Group 3, the endothelium da mage was not alleviated, and so were the NI. The Ce-H(2)O(2) particles deposite d on the lumen surface of endothelium were much less in Group 2 than in Groups 1 and 3. CONCLUSIONS: L-arginine promotes the repair process of IR-in jured endothelium probably through the removal of oxygen free radicals by NO.