Dysfunction of glutamatergic neurotransmission may be relevant to the pathogenesis of post-traumatic stress disorder (PTSD). Preclinical and clinical evidence suggests that PTSD symptoms could be alleviated following enhancement of neurotransmission mediated at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Eleven patients with chronic PTSD participated in a double-blind, placebo-controlled, cross-over trial with 50 mg/d D-cycloserine which acts as a partial agonist at the glycine regulatory site on the NMDA receptor. D-cycloserine treatment resulted in significant improvements in numbing, avoidance, and anxiety symptoms; however, similar effects were also observed during placebo treatment. In addition, D-cycloserine treatment resulted in a significant (p=0.03), reduction in the perseverative error scores as measured by the Wisconsin Card Sorting Test. This pilot study is the first to assess the efficacy of a NMDA receptor modulator for PTSD treatment and its results warrant further, larger-scale investigation.