Fear memories are critical for survival.Nevertheless,over-generalization of these memories,depicted by a failure to distinguish threats from safe stimuli,is typical in stress-related disorders.Previous studies have supported a protective role of ketamine against stress-induced depressive behavior.However,the effect of ketamine on fear generalization remains unclear.In this study,we investigated the effects of ketamine on fear generalization in a fear-generalized mouse model.The mice were given a single sub-anesthetic dose of ketamine (30 mg/kg,i.p.) 1 h before,1 week before,immediately after,or 22 h after fear conditioning.The behavioral measure of fear (indicated by freezing level) and synaptic protein expression in the basolateral amygdala (BLA) and inferior-limbic pre-frontal cortex (IL-PFC) of mice were examined.We found that only ketamine administered 22 h after fear conditioning significantly decreased the fear generalization,and the effect was dose-dependent and lasted for at least 2 weeks.The fear-generalized mice showed a lower level of brainderived neurotrophic factor (BDNF) and a higher level of GluN2B protein in the BLA and IL-PFC,and this was reversed by a single administration of ketamine.Moreover,the GluN2B antagonist ifenprodil decreased the fear generalization when infused into the IL-PFC,but had no effect when infused into the BLA.Infusion of ANA-12 (an antagonist of the BDNF receptor TrkB) into the BLA or IL-PFC blocked the effect of ketamine on fear generalization.These findings support the conclusion that a single dose of ketamine administered 22 h after fear conditioning alleviates the fear memory generalization in mice and the GluN2B-related BDNF signaling pathway plays an important role in the alleviation of fear generalization.