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 outpatient 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 participate in a laboratory session. This phase of the study will be conducted after at least 6 weeks of treatment while patients are on medication (prazosin or placebo).