The study design will consist of III phases. In phase I, all subjects will participate in three laboratory sessions to determine their reactivity to stress. Stress reactivity will be measured using: traumatic experiences, stressful non-trauma experiences and neutral experiences, presented randomly. Laboratory sessions will be conducted in an inpatient setting. Phase II is a randomized clinical trial evaluating prazosin versus placebo for 12 weeks in a double-blind, controlled fashion in an outpatient setting. The treatment will last for 12 weeks and outcomes will include symptoms of PTSD and alcohol use. In phase III, subjects will again be admitted to an inpatient unit. This phase of the study will be conducted during the 12th and final week on medication while participants are still receiving prazosin or placebo.