Post-traumatic stress disorder (PTSD) develops following an exposure to a life threatening event. One of the characteristic features of PTSD is the recurrence of intrusive memories of an experienced trauma. The persistence of disturbing traumatic memories in PTSD is often explained in terms of a trauma-induced enhancement of memory encoding. Several studies indicate that an increased noradrenergic activity during trauma enhances the encoding of memory. Elevated levels of norepinephrine in the cerebrospinal fluid of individuals with PTSD and the correlation of this elevation with the severity of PTSD symptoms suggest that increased noradrenergic activity is also involved in the maintenance of PTSD symptoms. Reactivation of memory by retrieval also renders the memory labile and susceptible to treatments. This latter process is referred to as memory reconsolidation. Consolidation and reconsolidation both occur within a distinct time window following new learning (in consolidation) and/or retrieval (in reconsolidation). Even well-consolidated old fear memories undergo reconsolidation and may be disrupted by means of pharmacological manipulation. Propranolol may be effective in treating PTSD long after symptoms have been consolidated. Propranolol given after reactivation of the memory of a past traumatic event reduces physiologic responding during subsequent mental imagery of the event.