Abstract: Objective:To observe the expression change of calmodulin kinaseⅡ(CaMK Ⅱ)in infra-limbic cortex of medial prefrontal cortex of adult rats in the early stage of fear extinction.Methods:Seventy rats were randomly divided into normal control,fear extinction and fear control group.The fear extinction training was not implemented in fear control group.The fear extinction group were trained to extinct fear with sound signal at 24 hours after fear established by sound hint of planter electric shock.The expressions of CaMK Ⅱ in infra-limbic cortex were detected by immunohistochemistry at 1,3 and 7 days after fear extinction.Results:The expression of CaMK Ⅱ in fear extinction and fear control group increased gradually,which was lower than that in normal control group.The fear extinction group were better than that in fear control group(P0.01).The immunoreactivities of CaMK Ⅱin infra-limbic cortex of fear extinction and fear control group increased gradually,which were lower than that in normal control group.The immunoreactivities of CaMK Ⅱ in fear extinction group were lower than that in fear control group at 3 and 7 days after fear extinction(P0.01),but no significant difference at day 1.Conclusions:The CaMK Ⅱ level in infra-limbic cortex is consistent with the extinction memory at 3 and 7 days after fear extinction training.