Patients with PTSD have small hippocampal volume and deficits in hippocampal-mediated memory as compared to healthy people. However, it is unclear whether the smaller hippocampi are a consequence of PTSD or a risk factor for the development of PTSD. Some researchers believe that people who develop PTSD have an increase in cortisol levels during traumatic experiences and that this could be neurotoxic to the hippocampus. Others hypothesize that increased sensitivity of glucocorticoid receptors to cortisol, regardless of the cortisol levels, could lead to neurotoxic damage to the hippocampus. This study will compare responses to a stress hormone in patients with PTSD, participants who have experienced trauma but do not have PTSD, and healthy volunteers.