Exposure and Response Prevention (ERP) therapy has become the treatment of choice for specific phobias. ERP involves systematic and repeated exposure to a feared or anxiety-provoking stimulus, leading to habituation and extinction of the fear response. Animal models of fear extinction have shown that acute administration of D-cycloserine (DCS) prior to exposure to a feared stimulus enhances extinction of that fear. A recent study in human subjects with height phobia (a specific phobia) has also demonstrated that DCS facilitates the effects of ERP therapy. Current theories postulate that DCS facilitates fear extinction by enhancing the learning process and increasing consolidation of memories, but the neural mechanisms underlying this process are not understood. The proposed research aims to elucidate these mechanisms by using fMRI to measure brain activation during 1) symptom provocation and verbal learning two hours post-medication, and 2)repeated symptom provocation and verbal recognition one week post-medication. This research will also examine the effects of DCS on cognitive functioning using neuropsychological testing both two house and one week post-medication.