Objective To determine the improved effect of methylene blue(MB)on cognitive func-tion in brain-inflammatory-aging rats and investigate the underlying mechanism.Methods A total of 38 healthy 12-month-old SD rats were randomly divided into healthy control group,lipopo-lysaccharide(LPS)group,MB vehicle group and MB group,with 8 rats in the control and 10 rats in the other three groups.LPS was injected into the fourth ventricle with aid of a subcutaneous sustained release pump to establish a rat model of brain chronic inflammatory aging.MB of 0.5 mg/(kg·d)was added into the pump in the rats from the MB group.T-maze test and new object recognition test were employed to evaluate the learning and memory abilities of the rats.The acti-vation of microglia and astrocytes in the hippocampal CA1 region of the rats was detected by im-munofluorescence assay.The release of inflammatory factors IL-1β and IL-6 was measured by ELISA,and neuronal death in the CA1 region was assessed by neuronal nuclei(NeuN)fluores-cence staining.Results There was no significant difference in the exploration time for new and old objects between the LPS group and the MB solvent group(P＞0.05).The MB group spent significantly longer time in exploring the new objects than the old object(22.50±4.32 s vs 11.60± 3.01 s,P=0.000).The alternating selection rate of new arm and expression level of NeuN antigen were significantly decreased,and the expression levels of ionized calcium binding adaptor mole-cule-1(Iba-1)and glial fibrillary acidic protein(GFAP)and the contents of IL-1β and IL-6 were obviously increased(P＜0.05)in the LPS group and the MB vehicle group than the healthy con-trol group.Compared with the MB vehicle group,the MB group had notably increased alternating selection rate of new arms and higher NeuN expression level,and decreased Iba-1 and GFAP ex-pression and IL-1β and IL-6 contents(P＜0.05).Conclusion Subcutaneous administration of MB could significantly inhibit the damages of spatial learning and memory abilities in the LPS-induced brain chronic inflammatory aging rats.The mechanism may be closely associated with MB inhibi-ting inflammatory glial cells and protecting hippocampal pyramidal neurons.