 A magnetic resonance imaging MRI scan will be done to create an image of the volunteers brain structure. The technique of MRI uses a strong magnetic field and radio waves to obtain images of body organs and tissues. During the MRI scan, volunteers will lie still on a table that will slide into the scanner for 30 minutes and in some cases up to but no more than 90 minutes. Volunteers will be asked to lie as still as possible during the procedure. Then a PET system will create two images of brain blood flow-one of brain dopamine 1 receptor and one of dopamine 23 receptor binding. Volunteers will be given a radiotracer, a tiny amount of a drug that can be detected by a special camera in the PET scanner. A tiny flexible tube will be placed in the vein of one arm during each PET scan but during the MRI scan. Volunteers will be asked to lie still on the PET scanner table. A mask with large holes for eyes, ears, and mouth will be placed over the head, to keep the head from moving. After radiotracer injections are given, the PET scanner will create brain images. There may be two PET scanning sessions, each requiring about 3 hours of scanning. During only one of these there will be breaks. At the end of the scanning session, volunteers will be asked to drink several glasses of water and urinate immediately, to reduce radiation exposure to the bladder wall. 