This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The purpose of the study is to look at reactions in the brain when a subject is expecting to receive a substance that causes a reaction in the brain similar to cocaine (methylphenidate) as opposed to the brain reaction when they actually receive methylphenidate. This will be done using Magnetic Resonance Imaging (MRI) to track a radiotracer (a radioactive chemical that is injected into the bloodstream) after the methylphenidate or a placebo is injected. Subjective measures will also be measured by asking cocaine abusers to report current feelings and mood throughout the study, and compare these subjective reports with the PET scans.