The study describes patterns of brain blood flow in post traumatic stress disorder (PTSD), using single photon emission computed tomography (SPECT). PTSD patients exhibit symptoms of abnormal memory, and it was hypothesized that abnormal memory formations are responsible for generation of PTSD symptoms. Brain blood flow changes, following activation of traumatic memories, might reflect this mechanism. Subjects are subjected to two types of auditory stimuli 48 hours apart--white noise and combat sounds--while their electrophysiological responses are monitored. 30mCi of 99m-Tc-HMPAO is injected after the termination of the stimulus. Acquisition is performed using a triple-headed Siemense gamma camera. Semiquantitative assessment and subtraction method are used in data analysis. Preliminary results reveal an increase in regional cerebral blood flow following the combat sounds activation (minus the white noise activation) in three regions of the brain. These regions were: 1) left and right parahippocampal gyrus, 2) left striatum, and 3) area of the upper brain stem structures. (Z score = 3.6, 3.2m and 3.1, respectively). The changes observed in parahippocampal gyri are of particular interest because of the proposed role of hippocampus and parahippocampal structures in formation, assessment, and storage of memory. Upper brain stem activation might reflect activation of centers responsible for increased arousal and autonomic responses.