Examine the interaction between stimulants, such as cocaine and methylphenidate, and impulsivity.

Inclusion Criteria:

1. age 18 - 50 years,
2. voluntary, written, informed consent,
3. physically healthy by medical history, physical, neurological, ECG, and laboratory examinations,
4. DSM-IV criteria for Cocaine Abuse (305.60) or Cocaine Dependence (304.20)
5. recent street cocaine use in excess of amounts to be administered in the current study,
6. intravenous and/or smoked (crack/ freebase) use,
7. positive urine toxicology screen for cocaine,
8. for females, non-lactating, no longer of child-bearing potential (or agree to practice effective contraception during the study), and a negative serum pregnancy (β-HCG) test.

Exclusion Criteria:

1. Other drug dependence (except nicotine) as determined by urine toxicology or interview
2. < 1 year of cocaine dependence,
3. a primary major DSM-IV psychiatric diagnosis (schizophrenia, bipolar disorder, etc.), unrelated to cocaine,
4. a history of significant medical (cardiovascular) or neurological illness, ie prior myocardial infarction, current active symptoms of cardiovascular disease / angina, evidence of cocaine-related cardiovascular symptoms, prior arrhythmias or need for cardiovascular resuscitation, neurovascular events such as transient ischemic attacks, stroke, and/or seizures Parameters re: elevations in vital signs are now explicitly specified under "Safety features built into our one-day self-administration paradigm).
5. current use of psychotropic and/or potentially psychoactive prescription medication,
6. seeking treatment for drug abuse/dependence (for experimental cocaine component),
7. physical or laboratory (β-HCG) evidence of pregnancy.
8. current use of any medication (prescription or over-the-counter) determined to cause potential drug interactions by the study physicians.