Effects of Psilocybin on Behavior, Psychology and Brain Function in Long-term Meditators

Inclusion criteria: * Have given written informed consent * Some college-level education (college degree preferred) * Be healthy and psychologically stable as determined by screening for medical and psychiatric problems via a personal interview, a medical questionnaire, a physical examination, an electrocardiogram (ECG), and routine medical blood and urinalysis laboratory tests * Agree to consume approximately the same amount of caffeine-containing beverage (e.g., coffee, tea) that he/she consumes on a usual morning, before arriving at the research unit on the mornings of drug session days. If the participant does not routinely consume caffeinated beverages, he/she must agree not to do so on session days. * Agree to refrain from using any psychoactive drugs, including alcoholic beverages and nicotine, within 24 hours of each drug administration. The exception is caffeine. Participants will be required to be non-smokers. * Agree not to take any Pro re-nata (PRN) medications on the mornings of drug sessions * Agree not to take sildenafil (Viagra®), tadalafil, or similar medications within 72 hours of each drug administration. * Agree that for one week before each drug session, he/she will refrain from taking any nonprescription medication, nutritional supplement, or herbal supplement except when approved by the study investigators. Exceptions will be evaluated by the study investigators and will include acetaminophen, non-steroidal anti-inflammatory drugs, and common doses of vitamins and minerals. * Live within driving distance of Baltimore - out-of-State residents may be eligible if they can provide their own transportation and lodging. * Have a long-term meditation practice, ideally in a Buddhist tradition, with good familiarity with breath meditation and loving kindness meditation; preference given to those with very long-term practices Exclusion criteria: * Women who are pregnant (as indicated by a positive urine pregnancy test assessed at intake and before each drug session) or nursing; women who are of child-bearing potential and sexually active who are not practicing an effective means of birth control. * Cardiovascular conditions: coronary artery disease, stroke, angina, uncontrolled hypertension, a clinically significant ECG abnormality (e.g., atrial fibrilation), or Transient Ischemic Attack (TIA) in the past year * Epilepsy with history of seizures * Insulin-dependent diabetes; if taking oral hypoglycemic agent, then no history of hypoglycemia * Currently taking psychoactive prescription medication on a regular (e.g., daily) basis * Currently taking on a regular (e.g., daily) basis any medications having a primary centrally-acting pharmacological effect on serotonin neurons or medications that are Monoamine Oxidase (MAO) inhibitors. For individuals who have intermittent or PRN use of such medications, psilocybin sessions will not be conducted until at least 5 half-lives of the agent have elapsed after the last dose. * More than 20% outside the upper or lower range of ideal body weight according to Metropolitan Life height and weight table * Head trauma * Claustrophobia * Cardiac pacemaker * Implanted cardiac defibrillator * Aneurysm brain clip * Inner ear implant * Artificial heart valve (last 6 weeks) * Prior history as a metal worker and/or certain metallic objects in the body Psychiatric Exclusion Criteria: * Current or past history of meeting Diagnostic and Statistical Manual (DSM)-IV criteria for Schizophrenia, Psychotic Disorder (unless substance-induced or due to a medical condition), or Bipolar I or II Disorder * Current or past history within the last 5 years of meeting DSM-IV criteria for alcohol or drug dependence (excluding caffeine and nicotine) or severe major depression * Have a first or second-degree relative with Schizophrenia, Psychotic Disorder (unless substance induced or due to a medical condition), or Bipolar I or II Disorder * Has a psychiatric condition judged to be incompatible with establishment of rapport or safe exposure to psilocybin