This study will enroll healthy participants. At visit 1 all participants will undergo a 1-hour startle test. Acoustic startle will be measured through assessing eye-blink response from the orbicularis oculi muscles in response to a 106 dB noise, using sensors under the eye. While watching the computer monitor, the participant will also experience several brief blasts of air directed at the throat. At visit 2, 24 hours later, participants will be randomized to receive 100 mg of MDMA or inactive placebo, after which they will undergo another startle task. The researchers will measure pulse, blood pressure and temperature once before and six times after receiving MDMA or placebo, and study participants will complete a questionnaire about their experience. At visit 3, participants will return for a final startle test and recall task. Sleep will be measured via self-report and Fittbit during study participation.