Abstract： Objective To study the mechanism of paraquat (PQ)-induced renal injury in rats,the expression changes of ICAM-1 to assess the protective effect of Melatonin in PQ poisoning.Methods Ninety adult healthy Sprague-Dawley (SD) rats were divided into three groups at random.Control group:30 rats;Poisoned group:30 rats;Melatonin group:30 rats.Control group and Poisoned group were treated intragastrically with 1 ml of PQ (50 mg/kg) diluted with normal saline.Control groupwere treated with the same dose of normal saline as Poisoned group and Melatonin group.Melatonin group were given 1 ml of Melatonin at a dose of 10 mg/kg diluted with normal saline (once daily,intraperitoneally) Control and Poisoned group were treated with the same dose of normal saline (once daily,intraperitoneally) as Melatonin group.Pathology of renal tissue were oberserved by HE staining,and electron microscope.The histopathological changes and the expression of ICAM-1 were observed with mmunohistochemistry (IHC).Results (1) There were no obvious pathological changes in Control group.Poisoned group Renal glomerulus had hyperemia and distension.Renal tubule epithelial cell had edema and vacuolar degeneration and renal tubule lumina was narrowing on day 1,There were serious edema exudation and necrosis on day 5,which gradually lessened furthermore;Compared with Poisoned group,the aforementioned pathological lesion was more palliative in Melatonin group.(2) No obvious abnomal changes in ultrastructure of renal tissues in Control group.There were swelling of mitochondrion and rupture of renal tubule epithelial cell and endoplasmic reticulum had extension,lysosome was mult and had much phagocytosis in Poisoned group.(3)There was a very weak expression of ICAM-1 in Control group.while in Poisoned group,there was already a significant higher expression of ICAM-1 of renal tubule on day 1 after PQ poisoning,Immunohistochemistry score (IHS) of Poisoned group on day 1,3,5,7,14 were (0.1561 ±0.0295、0.2572±0.0259、0.3028±0.0153、0.2083 ±0.0227、0.9309 ±0.0059),compared with Control group (P＜0.01);Melatonin group were (0.1259±0.0061、0.2109±0.0280、0.2679±0.0233、0.1771 ±0.0186、0.0791 ±0.0135),compared with Control group (P＜0.01),compared with Poisoned group (P＜0.05);Conclusion ICAM-1 was involved in the procedures of renal injury;MT surely had a protective effect,which might be mediated by ICAM-1 in the paraquat-induced renal injury,but its regulation path still need a further exploration.