ObjectiveTo explore the possible injury mechani sm in brain of rat irradiated by EMP through observing the changes in expression of CFOS in hippocampus and dentate gyrus.MethodsThe group A, group B and group C were irradiated by high field intensity(6104,2104 a nd 6~{!A~}103 V/m respectively)electromagnetic puls es(EMP)for 5 times within 2 minutes respectively and no treatment was given to g roup D(control group).The rats were sacrificed at 1, 6, 12,24 and 48 h,sep arately. Immunohistochemical method was used to investigate the expression of C FOS and image analysis for CFOS p ositive cells was performed by CMTAS~{*2r~} image analysis system at a magnificatio n of 400. Results The rats of irradiated groups had high expres sion of CFOS in neurons of hippocampus and dentate gyrus at 6-12 h, and the nu mber of CFOS positive cells was significantly increased as compared with contr ol group ~{*)~}(P0.05)~{**~}.Conclusions The EMP could cause significant high CFOS expression in hippocampus and gyrus,which may pla y an important role in the injury of rat brain by EMP.