"Cytokine theory of depression" indicated that cytokines induced by immune activation play an important role in depression.Many studies found that administration of LPS,an immunity-activation mediator,could induce depressive-like behavior,such as decrease of sweet milk preference,low locomotion,and anorexia.However,these researches only pay attention to the short-time behavior effects; the effects of LPS administration on long-time behavior changes have not been clearly reported.To further understand the role of cytokines induced by immunity-activation in depression,the purpose of the present study was to investigate the effects of repeated administration different doses of LPS on behavior and the long-time behavior effects in rats.