Inclusion Criteria:
* Males or females between the ages of 21-55 years. Females will be included if they are not pregnant and agreed to utilize a medically (non-hormonal)\* accepted birth control method (to include implant birth control, condom, diaphragm with spermicide, intrauterine device, tubal ligation, abstinence, or partner with vasectomy) or if post-menopausal for at least 1 year, or surgically sterile.
* Able to provide written informed consent according to Yale HIC guidelines.
* Able to read and write English as a primary language.
* Diagnosis of PTSD, as determined by the Clinician Administered PTSD Scale (CAPS-5).
* Must have a score of 23 or higher on the Clinician-Administered PTSD Scale (CAPS-5) at screening.
* No more than mild TBI according to a modified version of the Brief TBI Screen.
* Must not have a medical/neurological problem or use medication that would render MDMA unsafe by history or medical evaluation.
* No prior exposure to MDMA.
* Are willing to remain overnight at the study site after each experimental session.
* Are willing to be driven home the day after the experimental sessions.
* Not currently taking any of the listed medications at the time of the study.
* Are willing to sign a medical release for the investigators to communicate directly with their therapist and doctors.
* Are willing to abstain from alcohol, street drugs, and tobacco products while in the study.
Exclusion Criteria:
* Patients with a diagnostic history of bipolar disorder, schizophrenia or schizoaffective disorder or currently exhibiting psychotic features as determined by the MINI 7.0 for the DSM-5.
* Serious suicide or homicide risk, as assessed by evaluating clinician.
* Substance abuse or dependence during the 6 months prior to screening; or a positive pre-study (screening) urine drug screen.
* Any significant history of serious medical or neurological illness.
* Any signs of major medical or neurological illness on examination or as a result of ECG screening or laboratory tests (e.g. positive urine tox, positive HIV/AIDS tests ).
* Abnormality on physical examination. A participant with a clinical abnormality may be included only if the study physician considers the abnormality will not introduce additional risk factors and will not interfere with the study procedure.
* Pregnant or lactating women or a positive urine pregnancy test for women of child-bearing potential at screening or prior to any imaging day.
* Any history indicating learning disability, mental retardation, or attention deficit disorder.
* Family history of cardiovascular diseases. History of hypertension with baseline blood pressure above 140 mmHg (systolic) and over 90 mmHg (diastolic). Any history of syncope and/or baseline blood pressure below 100mmHg (systolic).
* History of claustrophobia.
* BMI \> 30 kg/m2 or \>250 pounds.
* Anxiolytic, neuroleptic and SRI medications (off SRIs for 4 weeks, fluoxetine 5 weeks).
* Females taking hormonal contraceptives will not be able to participate in the study \*(Hormonal contraceptives are exclusionary because MDMA increases production of oxytocin which is heavily modulated by other hormones (e.g. estrogen). Therefore, women need to be naturally cycling/ovulating and not taking any hormonal medications to participate in this study).
* Any metal or electromagnetic implants, including: (Cardiac pacemaker, artificial heart valve, defibrillator, aneurysm clip, cochlear implants, shrapnel, neurostimulators, history of metal fragments in eyes or skin, significant hearing loss or other severe sensory impairment, a history of seizures or current use of anticonvulsants.
