Participants in this study will be screened by telephone about their psychiatric and medical history, current emotional state, anxiety and sleep patterns, and family history of psychiatric disorders. At study entry, participants will have an interview, physical examination, electrocardiogram (EKG), and blood and laboratory tests. Menstruating women will have a pregnancy test and tests to determine menstrual phase and time of ovulation. At the second clinic visit, participants will undergo tests of intelligence and cognitive abilities and a magnetic resonance imaging (MRI) scan of the brain. Prior to Visits 3 and 4, participants will collect their saliva and urine. Menstruating women will have a pregnancy test. At Visits 3 and 4, participants will undergo TD studies and PET scanning. During one of these visits, participants will take capsules of an amino acid. On the other day, they will take lactose capsules. Throughout the study, participants will be asked about their emotional state, anxiety, ability to concentrate, and well being.