 The second goal of the study is to investigate if the administration of yohimbine, a drug found to promote the extinction of conditioned fear in animal models, and more recently, in humans with claustrophobia, improves the facilitation of fear extinction in PE. Yohimbine is a safe drug that is already extensively used in human populations. Specifically, this goal will be investigated using a double blind placebo controlled randomized trial design. The hypothesis is that one 21mg oral dose of yohimbine given concurrently with a 40 minute imaginal exposure exercise in PE will lead to a greater reduction in cue-induced anxiety during the following weekly PE session than placebo. This goal is significant because current projections of PTSD in OEFOIF veterans indicate that the need for psychological services will likely outpace the supply of such services. Accordingly, assisting treatments to be more efficient will likely translate into more veterans receiving much needed mental health services. 