Serotonin and oxytocin play a role in fear conditioning and fear extinction learning, psychological processes that are critically involved in psychiatric disorders such as posttraumatic stress disorder (PTSD). Specifically, administration of oxytocin has been shown to facilitate fear extinction in humans. Similarly, substances that release serotonin and oxytocin such as MDMA have been shown to enhance the extinction of fear memory in animals. However, there are no data on the effects of MDMA on fear extinction in humans. Therefore, the primary aim of this study is to investigate the role of acute serotonin release in the effects of fear extinction. MDMA will be used as pharmacological tool to induce serotonin release in this study.