A common approach to the clinical treatment of Posttraumatic Stress Disorder (PTSD) has focused on the facilitation of fear extinction through cognitive behavioural therapy that involves both safe exposure to the trauma-related cues and subsequent changes in conditioned stimulus–unconditioned stimulus (CS–US) contingency expectations. PTSD symptoms can be tracked back to pathologically modified associative fear, hyperarousal and a time-dependent fear generalization. We have used a mouse model of PTSD that is based on a brief exposure to an inescapable foot shock in order to investigate the influence of early (starting 1 day after the shock) and late (starting 1 month after the shock) extinction training. Both early and late extinction training led to a long-lasting reduction of contextual and generalized fear, but only early extinction caused an amelioration of hyperarousal. Consequently, our results suggest early post-shock intervention as a successful strategy for reducing hyperarousal in the aftermath of a trauma.