Objective To observre the change of plasma arginine vasopressin(AVP)in plasma and relationship of AVP with brain edema after traumatic brain injury(TBI).Methods The rat models of mild and severe degree local cerebral cortex injury were caused by free dropping.Radioimmunoassay was used to measure serial concentration of AVP in plasma in 45 rats with acute brain injuries at the 24th,48th,72th hour and the 5th day after brain injury.The content of injury brain water was measured by drying-wetting way.Results AVP levels had positive correlation with the severity of brain edema.The severer brain injury resulted in higher AVP level and longer persistent time of peak brain edema(P＜0.05).Conclusion AVP level is closely associated with edema after TBI.AVP may play a vital role in pathogenesis of brain edema.