Posttraumatic stress disorder(PTSD)is marked by hypermnesia of the trauma.Inhibition of the Rac GTPase has been recently suggested to impair the forgetting of initially acquired memory in Drosophila.Thus,we examined whether inhibition of Rac1 activity in the rat hippocampus could contribute to the hypermnesia of contextual fear.We found that spaced,but not massed training,of contextual fear conditioning caused inhibition of Rac1 activity and heightened contextual fear.Intrahippocampal injection of the Rac1 inhibitor NSC23766 in massed training or activator CN04-A in spaced training caused not only inhibition or activation of Rac1 activity in the hippocampus but also heightened or weakened contextual fear memory,respectively.Massed but not spaced extinction training immediately after retrieval caused robust Rac1 activity and more extinction of the contextual fear.Furthermore,the robust Rac1 activity by massed extinction training were also inhibited by intrahippocampal injection of NSC23766 and resulted in less extinction,while strong extinction training activated Rac1 in both massed and spaced groups and induced more extinction.Thus,we suggest that inhibition of Rac1 activity in the hippocampus regulates/impairs the forgetting of contextual fear memory and low Rac1 activity may lead to hypermnesia of the pathological fear in PTSD.