This study requires two overnight stays at the NIH Clinical Center. For each stay, subjects are admitted to the hospital the afternoon before the MRI scan. From the time of admission until after the scan, their diets are restricted to food relatively low in amino acids, prepared by NIH dietitians. The next day, participants undergo MRI scanning. For this procedure, the subject lies on a table that is moved into the scanner, a metal cylindrical machine. Earplugs are worn to muffle loud noises that occur with electrical switching of the radio frequency circuits. Imaging of brain structure takes about 10 to 15 minutes. Additional scans are then taken to measure brain activity while the subject plays simple computer games for money. These scans take about another 20 to 45 minutes. Five hours before the MRI, the subject drinks one of two beverages containing amino acids. A different beverage is given for each of the two visits: one drink lacks the essential amino acids tyrosine and phenylalanine, from which the body makes dopamine; the other contains balanced amounts of these two amino acids. Finally, subjects fill out mood-rating questionnaires before and after drinking each of the beverages.