Participants were recruited from the Omaha Veterans Affairs Medical Center following local IRB approval. We obtained written consent from all the subjects who were recruited in the study. Veterans between the ages of 19 and 65 years with chronic PTSD (diagnosis for >6 months) attributable to military combat exposure were included in the study. Patients were required to be clinically stable on their psychotropic medication regimen for at least three months prior to study entry. In addition to meeting DSM-IV criteria for PTSD and endorsing subjective complaints of memory difficulties, patients had to score least one standard deviation below the mean performance of a standardized, age- and sex-matched population on the Spatial Span, Logical Memory I, and Letter-Number Sequencing subtests of the Wechsler Memory Scale III (Third edition) for study entry. Patients with a history of dementia, schizophrenia, bipolar disorder, traumatic brain injury, and seizure were excluded. Patients with any history of alcohol or illicit drug abuse or dependence within the past one month were excluded. Patients requiring concomitant treatment with drugs with potential effects on the glutamergic system, such as amantadine,dextromethorphan, or carbonic anhydrase inhibitors, were excluded.