AIM: To explore the localization and semi-quantification o f the glo merular complement C5b-9 complexes and synthesis of some inflammatory mediators or cytokines such as nitric oxide (NO) and tumor necrosis factor ?(TNF?) in the ra ts with anti-thymocyte serum nephritis(ATSN). METHODS: The animal mo del of rat AT SN was reproduced by a single intravenous injection of anti-thymocyte serum (ATS ). Then, the deposits of glomerular C5b-9 complexes were localized and quantifie d by immunohistochemical staining and microscopic image scanning separately. And the glomerular mesangial cells (MC) surrounded by C5b-9 complexes were counted under microscope. In addition, the expression of glomerular MC inducible NO synt hase(iNOS) mRNA and excretion of urinary NO metabolite ( NO - 2/NO - 3 ) and TNF ? in the rats with ATSN were detected. RESULTS: The MC in t h e rats with ATSN emerged necrosis followed by a rapid proliferation. In the early time of MC injury, the C5b-9 complexes were mainly seen in glomerular mesangium and MC sur fac e. But with the progression of ATSN, the MC enclosed by C5b-9 appeared gr adual decrease. Moreover, the expression of MC iNOS mRNA in early stage of ATSN obviously increased and the excretion of urinary NO - 2/NO - 3 and TNF ? also significantly increased. However, the changes of parameters mentioned abov e in ATSN proliferative stage (after 7 days) alleviated gradually. CONCLUSION: The second ary lysis of MC has relation to the deposition of C5b-9 complexes and synthesis and release of NO and TNF ? in rats with ATSN.