Posttraumatic stress disorder (PTSD) is characterized by recurrent recollections and avoidance of the traumatic event and general hyperarousal that affect some victims of life-threatening events. In addition to the important application of animal fear conditioning paradigms in psychobiological studies of the mnemonic aspect of PTSD, special attention should be devoted to the exaggerated emotional and hormonal reactions to unconditioned stimuli, hallmarks of PTSD. Overreacting to harmless stimuli that do not bear resemblance to learned threats may be the result of a process of fear sensitization, which is vastly maladaptive and may interfere with daily life. It is argued that fear sensitization can possibly underpin the emergence of unconditioned behavioral disturbances seen in PTSD. Social withdrawal is also seen in PTSD, and physiological reactions may exacerbate behavioral avoidance to social situations. In this regard, we have evaluated fear conditioning and several behavioral indexes of fear sensitization, characteristic of posttraumatic stress in rats. We hypothesized that exposure to novel social stimuli might be associated with high hormonal levels in rats previously submitted to an unpredictable and inescapable highly intense electric footshock, a model of severe stress. We found that, in addition to a robust contextual fear conditioning and a consistent behavioral overreactive state, the hormonal responsiveness to a socioenvironmental stimulus was also exacerbated in rats exposed to the traumatic stress, compared with control rats. Our data corroborate previous studies showing affective sequalae in PTSD animal models and provide additional information on physiological reactions associated with traumatic stress in rats.